VANGUARD INSTITUTIONAL INDEX FUND
485BPOS, 1997-06-24
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<PAGE>   1
 
================================================================================
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
 
                                   FORM N-1A
                  REGISTRATION STATEMENT (NO. 33-34494) UNDER
                           THE SECURITIES ACT OF 1933
   
                        POST-EFFECTIVE AMENDMENT NO. 11
    
                                      AND
 
              REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY
                                  ACT OF 1940
   
                                AMENDMENT NO. 12
    
                             VANGUARD INSTITUTIONAL
                                   INDEX FUND
               (EXACT NAME OF REGISTRANT AS SPECIFIED IN CHARTER)
 
                     100 VANGUARD BOULEVARD, P.O. BOX 2600,
                             MALVERN, PA 19355-0741
                    (ADDRESS OF PRINCIPAL EXECUTIVE OFFICE)
                  REGISTRANT'S TELEPHONE NUMBER (610) 669-1000
 
                         RAYMOND J. KLAPINSKY, ESQUIRE
                                  P.O. BOX 876
                             VALLEY FORGE, PA 19482
 
                IT IS PROPOSED THAT THIS FILING BECOME EFFECTIVE
   
             on July 7, 1997 pursuant to paragraph (b) of Rule 485.
    
 
                 APPROXIMATE DATE OF PROPOSED PUBLIC OFFERING:
  As soon as practicable after this Registration Statement becomes effective.
 
     WE HAVE ELECTED TO REGISTER AN INDEFINITE NUMBER OF SHARES PURSUANT TO RULE
24F-2 UNDER THE INVESTMENT COMPANY ACT OF 1940. WE FILED OUR RULE 24F-2 NOTICE
FOR THE YEAR ENDED DECEMBER 31, 1996 ON FEBRUARY 27, 1997.
 
================================================================================
<PAGE>   2
 
                       VANGUARD INSTITUTIONAL INDEX FUND
 
                             CROSS REFERENCE SHEET
<TABLE>
<CAPTION>
FORM N-1A
ITEM NUMBER                                                           LOCATION IN PROSPECTUS
<C>           <S>                                              <C>
    Item 1.   Cover Page....................................   Cover Page
    Item 2.   Synopsis......................................   Fund and Trust Expenses
    Item 3.   Condensed Financial Information...............   Fund Expenses, Financial Highlights
    Item 4.   General Description of Registrant.............   Investment Objective; Investment
                                                               Limitations; Investment Policies;
                                                               Investment Risks; General Information
    Item 5.   Management of the Fund........................   Management and Investment Advisory
                                                               Services
   Item 5A.   Management's Discussion of Fund Performance...   Herein incorporated by reference to
                                                               Registrant's Annual Report to
                                                               Shareholders dated December 31, 1996
                                                               filed with the Securities and
                                                               Exchange Commission's EDGAR system on
                                                               February 20, 1997
    Item 6.   Capital Stock and Other Securities............   Opening an Account and Purchasing
                                                               Shares; Selling Shares; The Share
                                                               Price of the Fund and Each Portfolio;
                                                               Dividends, Capital Gains and Taxes;
                                                               General Information
    Item 7.   Purchase of Securities Being Offered..........   Opening an Account and Purchasing
                                                               Shares; Exchanging Shares; Exchange
                                                               Privilege Limitations
    Item 8.   Redemption or Repurchase......................   Selling Shares
    Item 9.   Pending Legal Proceedings.....................   Not Applicable
 
<CAPTION>
FORM N-1A                                                              LOCATION IN STATEMENT
ITEM NUMBER                                                          OF ADDITIONAL INFORMATION
<C>           <S>                                              <C>
   Item 10.   Cover Page....................................   Cover Page
   Item 11.   Table of Contents.............................   Cover Page
   Item 12.   General Information and History...............   Not Applicable
   Item 13.   Investment Objectives and Policies............   Investment Objective and Policies;
                                                               Investment Limitations
   Item 14.   Management of the Registrant..................   Management and Advisory Services
   Item 15.   Control Persons and Principal Holders
              of Securities.................................   Not Applicable
   Item 16.   Investment Advisory and Other Services........   Management and Advisory Services
   Item 17.   Brokerage Allocation and Other Practices......   Portfolio Transactions
   Item 18.   Capital Stock and Other Securities............   Description of Shares and Voting
                                                               Rights
   Item 19.   Purchase, Redemption and Pricing of Securities
              Being Offered.................................   Purchase of Shares; Redemption of
                                                               Shares
   Item 20.   Tax Status....................................   Not Applicable
   Item 21.   Underwriters..................................   Not Applicable
   Item 22.   Calculations of Performance Data..............   Yield and Total Return; Performance
                                                               Measures
   Item 23.   Financial Statements..........................   Financial Statements
</TABLE>
<PAGE>   3
 
                                                                     [FLAG LOGO]
                         P  R  O  S  P  E  C  T  U  S 
   
                                  JULY 7, 1997
    
 
                       VANGUARD INSTITUTIONAL INDEX FUND

                              Institutional Shares
                           Institutional Plus Shares

                              VANGUARD INDEX TRUST
                              Institutional Shares
                                       of

                           EXTENDED MARKET PORTFOLIO

                          TOTAL STOCK MARKET PORTFOLIO

                      SMALL CAPITALIZATION STOCK PORTFOLIO
<PAGE>   4
 
================================================================================
 
   
PROSPECTUS -- JULY 7, 1997
    
================================================================================
 
   
FUND INFORMATION: 1-800-523-1036
    
- --------------------------------------------------------------------------------
 
INVESTMENT
OBJECTIVES AND
POLICIES               The Vanguard index funds (the "Funds") offered in this
                       prospectus are Vanguard Institutional Index Fund and
                       three Portfolios of Vanguard Index Trust (the Extended
                       Market, Total Stock Market and Small Capitalization Stock
                       Portfolios). The Funds are open-end, diversified
                       investment companies. Each Fund invests in common stocks
                       in order to match the investment performance of a
                       distinct market index. There is no assurance that the
                       Funds will achieve their stated objectives. Shares of the
                       Funds are neither insured nor guaranteed by any agency of
                       the U.S. Government, including the FDIC.
- --------------------------------------------------------------------------------
 
   
INVESTMENT
ALTERNATIVES           VANGUARD INSTITUTIONAL INDEX FUND offers two, separate
                       classes of shares. The "Institutional Shares" are
                       designed for investors who meet the minimum initial
                       investment of $10 million and may require special
                       employee benefit plan services. The "Institutional Plus
                       Shares" are designed for investors who meet the minimum
                       initial investment of $200 million and do not require
                       special employee benefit plan services. This prospectus
                       relates to both the Institutional Shares and the
                       Institutional Plus Shares of Vanguard Institutional Index
                       Fund.
    
 
   
                       VANGUARD INDEX TRUST EXTENDED MARKET PORTFOLIO, TOTAL
                       STOCK MARKET PORTFOLIO, AND SMALL CAPITALIZATION STOCK
                       PORTFOLIO each offers two, separate classes of shares.
                       This prospectus relates to the "Institutional Shares" of
                       these Funds, which are designed for investors who meet
                       the minimum initial investment of $10 million and
                       generally do not require special employee benefit plan
                       services. The "Portfolio Shares" of the Funds feature a
                       $3,000 minimum initial investment and are offered by a
                       separate prospectus. To obtain information on the
                       Portfolio Shares, please call 1-800-662-7447 (SHIP),
                       Monday through Friday, from 8:00 a.m. to 9:00 p.m. and
                       Saturday from 9:00 a.m. to 4:00 p.m.
    
 
   
                       Each Fund's share classes have different expenses; as a
                       result, their investment performance will vary.
    
- --------------------------------------------------------------------------------
 
   
OPENING AN
ACCOUNT                Shares of the Funds may be purchased by Federal Funds
                       wire of the minimum initial investment. In certain
                       circumstances, a portfolio transaction fee may be
                       deducted from purchases of Vanguard Institutional Index
                       Fund and the Total Stock Market Portfolio. A portfolio
                       transaction fee of 0.5% is deducted from purchases of the
                       Extended Market and Small Capitalization Stock
                       Portfolios. These fees offset the Funds' transaction
                       costs of buying and selling securities.
    
- --------------------------------------------------------------------------------
 
   
ABOUT THIS
PROSPECTUS             This prospectus is designed to set forth concisely the
                       information you should know about the Funds before you
                       invest. It should be retained for future reference.
                       "Statements of Additional Information" containing
                       additional information about Vanguard Institutional Index
                       Fund, dated July 7, 1997 and Vanguard Index Trust, dated
                       July 7, 1997, have been filed with the Securities and
                       Exchange Commission. These Statements have been
                       incorporated by reference into this prospectus. Copies
                       may be obtained, along with other information about the
                       Funds, without charge by contacting Vanguard or visiting
                       the Securities and Exchange Commission's web site
                       (http.//www.sec.gov).
    
- --------------------------------------------------------------------------------
 
TABLE OF CONTENTS
 
   
<TABLE>
<S>                            <C>       <C>                            <C>       <C>                            <C>
Highlights...................    2       Implementation of Policies...   14               SHAREHOLDER GUIDE
Fund Expenses................    4       Investment Limitations.......   19       Opening an Account and
Financial Highlights.........    6       Management and Investment                  Purchasing Shares..........     26
Yield and Total Return.......   10         Advisory Services..........   19       Trade Date Policy............     28
        FUND INFORMATION                 Portfolio Transactions.......   21       Selling Your Shares..........     29
Investment Objectives........   10       Dividends, Capital Gains                 Exchanging Your Shares.......     30
Investment Policies..........   11         and Taxes..................   22       Exchange Privilege                  
Investment Risks.............   13       The Share Price of Each                    Limitations................     30 
Who Should Invest............   14         Fund.......................   23       Important Information About         
                                         General Information..........   24         Telephone Transactions.....     30
                                                                                  Other Account Information....     31
</TABLE>
    
 
- --------------------------------------------------------------------------------
 
THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION, NOR HAS THE SECURITIES
AND EXCHANGE COMMISSION OR ANY STATE COMMISSION PASSED UPON THE ACCURACY OR
ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL
OFFENSE.
- --------------------------------------------------------------------------------
<PAGE>   5
 
                                   HIGHLIGHTS
 
   
                        This prospectus describes Vanguard Institutional Index
                        Fund (Institutional and Institutional Plus Shares) and
                        Vanguard Index Trust -- Extended Market Portfolio, Total
                        Stock Market Portfolio and Small Capitalization Stock
                        Portfolio (Institutional Shares, only). Unlike other
                        mutual funds which generally attempt to "beat" market
                        averages with often unpredictable results, each of these
                        Funds seeks to "match" the performance of a different
                        stock market benchmark or index. The Funds are expected
                        to provide highly predictable returns relative to their
                        benchmarks. The Funds offer investors the advantages of
                        a "passive" approach to investing. These include low
                        investment costs, broad diversification among
                        securities, minimal portfolio turnover, and relative
                        predictability.
    
 
                        As with any mutual fund there is no assurance that the
                        Funds will meet their objectives.
- --------------------------------------------------------------------------------
 
INVESTMENT
OBJECTIVES AND
POLICIES                The Funds are open-end, diversified investment companies
                        designed as "index" funds.
 
VANGUARD INSTITUTIONAL
INDEX FUND              The Vanguard Institutional Index Fund seeks to replicate
                        the performance of the S&P 500 Index.
  ------------------------------------------------------------------------------
 
VANGUARD INDEX TRUST    Vanguard Index Trust consists of six separate
                        Portfolios, each of which invests in U.S. common stocks.
                        The Institutional Shares of three Vanguard Index Trust
                        Portfolios are offered through this prospectus:
 
                        - The EXTENDED MARKET PORTFOLIO seeks to replicate the
                          performance of the Wilshire 4500 Index.
                        - The TOTAL STOCK MARKET PORTFOLIO seeks to replicate
                          the performance of the Wilshire 5000 Index.
                        - The SMALL CAPITALIZATION STOCK PORTFOLIO seeks to
                          replicate the performance of the Russell 2000 Small
                          Stock Index.                                   PAGE 14
- --------------------------------------------------------------------------------
 
INVESTMENT
RISKS                   The Funds are subject to stock market risk, which is the
                        possibility that common stock prices will decline over
                        short or extended periods. U.S. stock markets tend to be
                        cyclical, with periods when stock prices generally rise
                        and periods when stock prices generally decline.
 
                        Because of the risks associated with common stocks, the
                        Funds are intended to be long-term investment vehicles
                        and are not designed to provide investors with a means
                        of speculating on short-term market movements. Investors
                        should not consider an investment in any one portfolio a
                        complete investment program, but should maintain
                        holdings of securities with different risk
                        characteristics -- including U.S. common stocks, bonds
                        and money market instruments. For further information
                        concerning the risks associated with investing in the
                        Funds, see "Investment Risks."                   PAGE 17
- --------------------------------------------------------------------------------
 
                                        2
<PAGE>   6
 
THE VANGUARD
GROUP                   Vanguard Index Trust is a member of The Vanguard Group
                        of Investment Companies, a group of more than 30
                        investment companies with more than 90 distinct
                        investment portfolios and total assets in excess of $250
                        billion. The Vanguard Group, Inc. ("Vanguard"), a
                        jointly owned subsidiary of the Vanguard funds, provides
                        all corporate management, administrative, distribution
                        and shareholder accounting services on an at-cost basis
                        to the funds in the Group. Vanguard Institutional Index
                        Fund is not part of the Group, however, it employs
                        Vanguard to provide virtually all of its necessary
                        services.                                        PAGE 23
- --------------------------------------------------------------------------------
 
   
INVESTMENT
ADVISER                 The Funds receive investment advisory services from
                        Vanguard's Core Management Group.                PAGE 23
    
- --------------------------------------------------------------------------------
 
FEES AND EXPENSES       The following transaction fees apply to share purchases:
 
   
<TABLE>
<CAPTION>
                                                                                     FEE DEDUCTED
                                                                                    FROM PURCHASES
                           <S>                                                      <C>
                           Vanguard Institutional Index Fund                             None*
                           Vanguard Extended Market Portfolio                              .5%
                           Vanguard Total Stock Market Portfolio                         None*
                           Vanguard Small Capitalization Stock Portfolio                   .5%
</TABLE>
    
 
   
                       * Under certain circumstances, Vanguard Institutional
                         Index Fund and Vanguard Total Stock Market Portfolio
                         may deduct a portfolio transaction fee, ranging from
                         0.08% to 0.20%, from purchases of their shares.
    
 
                        Where portfolio transaction fees apply, they are paid
                        directly to the Fund to offset transaction costs of
                        buying securities.
 
   
                                                                          PAGE 5
    
- --------------------------------------------------------------------------------
 
                                        3
<PAGE>   7
 
   
FUND EXPENSES           The following tables illustrate ALL expenses and fees
                        that you would incur as a shareholder of each share
                        class of Vanguard Institutional Index Fund and Vanguard
                        Index Trust -- Extended Market Portfolio, Total Stock
                        Market Portfolio and Small Capitalization Portfolio.
                        Because the Institutional Plus Shares of Vanguard
                        Institutional Index Fund and the Institutional Shares of
                        the other Funds were not offered prior to July 7, 1997,
                        the table is based on the historical expenses of these
                        Funds' other share classes for the 1996 fiscal year. The
                        expenses and fees for the Institutional Shares of
                        Vanguard Institutional Index Fund and the Portfolio
                        Shares of the other Funds are also based upon those
                        incurred during the 1996 fiscal year.
    
 
<TABLE>
<CAPTION>
                                                             INSTITUTIONAL                   INSTITUTIONAL   INSTITUTIONAL
                                                                 PLUS        INSTITUTIONAL     SHARES OF       SHARES OF
                                                               SHARES OF       SHARES OF         TOTAL           SMALL
                              SHAREHOLDER                      VANGUARD        EXTENDED          STOCK       CAPITALIZATION
                              TRANSACTION                    INSTITUTIONAL      MARKET          MARKET           STOCK
                              EXPENSES                        INDEX FUND       PORTFOLIO       PORTFOLIO       PORTFOLIO+
                              ---------------------------------------------------------------------------------------------
                              <S>                            <C>             <C>             <C>             <C>
                              Sales Load Imposed on
                                Purchases..................       None*           None**          None*           None**
                              Sales Load Imposed on
                                Reinvested Dividends.......       None            None            None            None
                              Redemption Fees..............       None            None            None            None
                              Exchange Fees................       None            None            None            None
</TABLE>
 
   
<TABLE>
<CAPTION>
                                                             INSTITUTIONAL                   INSTITUTIONAL   INSTITUTIONAL
                                                                 PLUS        INSTITUTIONAL     SHARES OF       SHARES OF
                                                               SHARES OF       SHARES OF         TOTAL           SMALL
                                                               VANGUARD        EXTENDED          STOCK       CAPITALIZATION
                              ANNUAL FUND                    INSTITUTIONAL      MARKET          MARKET           STOCK
                              OPERATING EXPENSES              INDEX FUND       PORTFOLIO       PORTFOLIO       PORTFOLIO+
                              ---------------------------------------------------------------------------------------------
                              <S>                            <C>             <C>             <C>             <C>
                              Management and Administrative
                                Expenses...................      .025%            .06%            .06%            .08%
                              Investment Advisory Fees.....       None            None            None            None
                              12b-1 Fees...................       None            None            None            None
                              Other Expenses
                                Distributions Costs........       None            .02%            .02%            .02%
                                Miscellaneous Expenses.....       None            .02%            .02%            .02%
                                                                 -----           -----           -----           -----
                                      Total Operating
                                        Expenses...........      .025%            .10%            .10%            .12%
                                                                 =====           =====           =====           =====
</TABLE>
    
 
                        * As described below, these Funds reserve the right to
                          deduct a portfolio transaction fee, ranging from 0.08%
                          to 0.20% from purchases of their shares.
 
                       ** As described below, these Funds deduct a 0.5%
                          portfolio transaction fee from each purchase of
                          shares.
 
                        + Formerly Vanguard Small Capitalization Stock Fund,
                          Inc.
 
                                        4
<PAGE>   8
 
<TABLE>
<CAPTION>
                                                                                               PORTFOLIO       PORTFOLIO
                                                             INSTITUTIONAL     PORTFOLIO       SHARES OF       SHARES OF
                                                               SHARES OF       SHARES OF         TOTAL           SMALL
                              SHAREHOLDER                      VANGUARD        EXTENDED          STOCK       CAPITALIZATION
                              TRANSACTION                    INSTITUTIONAL      MARKET          MARKET           STOCK
                              EXPENSES                        INDEX FUND       PORTFOLIO       PORTFOLIO       PORTFOLIO+
                              ----------------------------------------------------------------------------------------------
                              <S>                            <C>             <C>             <C>             <C>
                              Sales Load Imposed on
                                Purchases..................       None*           None**          None            None**
                              Sales Load Imposed on
                                Reinvested Dividends.......       None            None            None            None
                              Redemption Fees..............       None            None            None            None
                              Exchange Fees................       None            None            None            None
</TABLE>
 
<TABLE>
<CAPTION>
                                                                                               PORTFOLIO       PORTFOLIO
                                                             INSTITUTIONAL     PORTFOLIO       SHARES OF       SHARES OF
                                                               SHARES OF       SHARES OF         TOTAL           SMALL
                                                               VANGUARD        EXTENDED          STOCK       CAPITALIZATION
                              ANNUAL FUND                    INSTITUTIONAL      MARKET          MARKET           STOCK
                              OPERATING EXPENSES              INDEX FUND       PORTFOLIO       PORTFOLIO       PORTFOLIO+
                              ----------------------------------------------------------------------------------------------
                              <S>                            <C>             <C>             <C>             <C>
                              Management & Administrative
                                Expenses...................       0.06%           0.21%           0.18%           0.21%
                              Investment Advisory Fees.....       None            None            None            None
                              12b-1 Fees...................       None            None            None            None
                              Other Expenses
                                Distributions Costs........       None            0.02%           0.02%           0.02%
                                Miscellaneous Expenses.....       None            0.02%           0.02%           0.02%
                                                                 -----           -----           -----           -----
                                  TOTAL OPERATING
                                    EXPENSES...............       0.06%           0.25%           0.22%           0.25%
                                                                 =====           =====           =====           =====
</TABLE>
 
                        *As described below, the Fund reserves the right to
                         deduct a portfolio transaction fee, ranging from 0.08%
                         to 0.20%, from purchases of its shares.
                       **As described below, these Funds deduct a 0.5% portfolio
                         transaction fee from each purchase of shares.
                        +Formerly Vanguard Small Capitalization Stock Fund, Inc.
 
- --------------------------------------------------------------------------------
 
   
PORTFOLIO
TRANSACTION
FEES                    Vanguard Institutional Index Fund and Vanguard Index
                        Trust Total Stock Market Portfolio reserve the right to
                        deduct a portfolio transaction fee, ranging from 0.08%
                        to 0.20%, from purchases of their shares, if any one
                        purchase or cumulative purchases are of a size that is
                        reasonably deemed to be disruptive to efficient
                        portfolio management. A prospective investor may
                        determine whether this fee will apply by calling
                        Vanguard's Institutional Investor Services Department at
                        1-800-523-1036 before purchasing shares.
    
 
   
                        Vanguard Index Trust -- Extended Market Portfolio and
                        Small Capitalization Stock Portfolio each assess a
                        portfolio transaction fee equal to 0.5% of the dollar
                        amount invested. All portfolio transaction fees are paid
                        to the respective Fund, not to Vanguard. They are not
                        sales charges.
    
 
                        These fees apply to initial investments in the
                        respective Funds and all subsequent purchases (including
                        purchases made by exchange from another Vanguard fund),
                        but not to reinvested dividend or capital gains
                        distributions. Portfolio transaction fees are deducted
                        automatically from the amount invested; they cannot be
                        paid separately.
 
   
                        The purpose of these fees is to allocate transaction
                        costs associated with new purchases to investors making
                        those purchases, thus insulating existing shareholders
                        from those transaction costs. These costs include: (1)
                        brokerage costs; (2) market impact costs -- i.e., the
                        increase in market prices which may result when a Fund
                        purchases thinly traded stocks; and, most importantly,
                        (3) the effect of the "bid-ask" spread in the
                        over-the-counter market. (Securities in the
                        over-the-counter market
    
 
                                        5
<PAGE>   9
 
   
                        are bought at the "ask" or purchase price, but are
                        valued in a Fund at the mean of the "bid" or sale, and
                        "ask" prices).
    
 
   
                        The fees represent Vanguard's estimate of the
                        transaction costs incurred by the Funds in acquiring
                        stocks in their respective markets. Without the fees,
                        the Funds, which incur these costs directly, would
                        experience reduced investment performance for all of
                        their shareholders. With the fees, the transaction costs
                        of acquiring additional stocks are borne not by all
                        existing shareholders, but by those investors making
                        additional purchases. Because the purchaser, not the
                        Funds, bears these costs, the Funds are expected to
                        track their respective benchmark indexes more closely.
                        The purpose of these tables is to assist an investor in
                        understanding the various expenses that an investor in
                        each Fund would bear directly or indirectly.
    
 
                        The following example illustrates the expenses that an
                        investor would incur on a $1,000 investment over various
                        periods, assuming (1) a 5% annual rate of return; and
                        (2) redemption at the end of each period. As noted in
                        the table above, there are no redemption fees of any
                        kind.
 
   
<TABLE>
<CAPTION>
                                                                         1       3       5      10
                                                                        YEAR   YEARS   YEARS   YEARS
                           -------------------------------------------------------------------------
                           <S>                                          <C>    <C>     <C>     <C>
                           VANGUARD INSTITUTIONAL INDEX FUND
                             Institutional Plus Shares................  $ 0     $ 1     $ 1     $ 3
                             Institutional Shares.....................  $ 1     $ 2     $ 3     $ 8
                           VANGUARD INDEX TRUST
                             EXTENDED MARKET PORTFOLIO
                             Institutional Shares.....................  $ 6     $ 8     $11     $18
                             Portfolio Shares.........................  $ 8     $13     $19     $37
                             TOTAL STOCK MARKET PORTFOLIO
                             Institutional Shares.....................  $ 1     $ 3     $ 6     $13
                             Portfolio Shares.........................  $ 2     $ 7     $12     $28
                             SMALL CAPITALIZATION STOCK PORTFOLIO
                             Institutional Shares.....................  $ 6     $ 9     $12     $20
                             Portfolio Shares.........................  $ 8     $13     $19     $37
</TABLE>
    
 
   
                        These estimates include an account maintenance fee of
                        $10 per year for the Portfolio Shares of the Extended
                        Market Portfolio, Total Stock Market Portfolio and Small
                        Capitalization Stock Portfolio. This fee applies to
                        accounts with balances less than $10,000 only.
    
 
   
                        Any relevant transaction fees are included in these
                        estimates; the $10 per year account maintenance fees are
                        not.
    
 
                        THIS EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION
                        OF PAST OR FUTURE EXPENSES OR PERFORMANCE. ACTUAL
                        EXPENSES MAY BE HIGHER OR LOWER THAN THOSE SHOWN.
- --------------------------------------------------------------------------------
 
   
FINANCIAL
HIGHLIGHTS              The following financial highlights for a share
                        outstanding throughout each period presented have been
                        audited by Price Waterhouse LLP, independent
                        accountants, whose reports on the financial statements,
                        which include this information, were unqualified. This
                        information should be read in conjunction with the
                        financial statements and notes thereto appearing in
                        Vanguard Institutional Index Fund's 1996 Annual
    
 
                                        6
<PAGE>   10
 
   
                        Report to Shareholders and Vanguard Index Trust's 1996
                        Annual Report to Shareholders relating to the Extended
                        Market, Total Stock Market and Small Capitalization
                        Stock Portfolios, which are incorporated by reference in
                        the Funds' Statements of Additional Information and this
                        Prospectus, and which appear, along with the reports of
                        Price Waterhouse LLP, in the Funds' 1996 Annual Reports
                        to Shareholders. For a more complete discussion of the
                        Funds' performance, please see the Funds' 1996 Annual
                        Reports to Shareholders, which may be obtained without
                        charge by writing to or calling Vanguard
                        (1-800-523-1036).
    
 
   
                        These financial highlights relate only to the
                        Institutional Shares of Vanguard Institutional Index
                        Fund and the Portfolio Shares of the other Funds; each
                        Fund's other share class was not in existence during the
                        periods covered here.
    
 
   
<TABLE>
<CAPTION>
                                --------------------------------------------------------------------------------
                                           VANGUARD INSTITUTIONAL INDEX FUND -- INSTITUTIONAL SHARES
                                --------------------------------------------------------------------------------
                                               YEAR ENDED DECEMBER 31,
                             -----------------------------------------------------------        JULY 31, 1990*
                             1996        1995       1994       1993       1992       1991      TO DEC. 31, 1990
<S>                         <C>         <C>        <C>        <C>        <C>        <C>        <C>             
- ------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, BEGINNING
  OF PERIOD...............   $57.93     $43.22     $44.20     $41.45     $39.91     $31.62          $34.10
                            -------     ------     ------     ------     ------     ------     ------------
INVESTMENT OPERATIONS
  Net Investment Income...     1.38       1.28       1.23       1.20       1.17       1.16             .52
  Net Realized and
    Unrealized Gain (Loss)
    on Investments........    11.90      14.86       (.66)      2.92       1.79       8.35           (2.48)
                             ------     ------     ------     ------     ------     ------     -----------
    TOTAL FROM INVESTMENT
      OPERATIONS..........    13.28      16.14        .57       4.12       2.96       9.51           (1.96)
- ------------------------------------------------------------------------------------------------------------
DISTRIBUTIONS
  Dividends from Net
    Investment Income.....    (1.36)     (1.27)     (1.21)     (1.19)     (1.17)     (1.16)           (.52)
  Distributions from
    Realized Capital
    Gains.................     (.99)      (.16)      (.34)      (.18)      (.25)      (.06)             --
                             ------     ------     ------     ------     ------     ------     -----------
    TOTAL DISTRIBUTIONS...    (2.35)     (1.43)     (1.55)     (1.37)     (1.42)     (1.22)           (.52)
- ------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, END OF
  PERIOD..................   $68.86     $57.93     $43.22     $44.20     $41.45     $39.91          $31.62
- ------------------------------------------------------------------------------------------------------------
TOTAL RETURN..............    23.06%     37.60%      1.31%     10.02%      7.54%     30.34%          (5.74)%
- ------------------------------------------------------------------------------------------------------------
RATIOS/SUPPLEMENTAL DATA
Net Assets, End of Period
  (Millions)..............  $11,426     $6,674     $3,265     $3,103     $1,525     $1,069            $512
Ratio of Total Expenses to
  Average Net Assets......     0.06%      0.06%      0.07%      0.07%      0.07%      0.08%           0.09%**
Ratio of Net Investment
  Income to Average Net
  Assets..................     2.18%      2.49%      2.80%      2.72%      2.94%      3.15%           3.98%**
Portfolio Turnover Rate...        9%         4%+       23%+        4%+        9%+        4%              2%
Average Commission Rate
  Paid....................   $.0167        N/A        N/A        N/A        N/A        N/A             N/A
   * Commencement of operations.
  ** Annualized.
   + The portfolio turnover rates excluding in-kind redemptions were 4%, 19%, 3%, and 6%, respectively.
</TABLE>
    
 
                                        7
<PAGE>   11
 
   
<TABLE>
<CAPTION>
                         ----------------------------------------------------------------------------------------------------
                                                       EXTENDED MARKET PORTFOLIO -- PORTFOLIO SHARES
                         ----------------------------------------------------------------------------------------------------
                                                        YEAR ENDED DECEMBER 31,
                         -------------------------------------------------------------------------------------
                                                                                                                   DEC. 21,+
                          1996      1995      1994      1993      1992      1991      1990      1989      1988    TO 31, 1987
<S>                      <C>       <C>       <C>       <C>       <C>       <C>       <C>       <C>       <C>      <C>       
- -----------------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE,
 BEGINNING OF
 PERIOD...............   $24.07    $18.52    $19.43    $17.35    $15.82    $11.48    $13.92    $11.60     $9.99      $10.00
                          -----     -----     -----    ------    ------    ------    ------    ------    ------    --------
INVESTMENT OPERATIONS
 Net Investment
   Income.............      .34       .30       .28       .23       .24       .25       .30       .26       .34         .03
 Net Realized and
   Unrealized Gain
   (Loss) on
   Investments........     3.85      5.95      (.62)     2.28      1.72      4.54     (2.25)     2.52      1.63        (.04)
                          -----     -----     -----    ------    ------    ------    ------    ------    ------    --------
   TOTAL FROM
     INVESTMENT
     OPERATIONS.......     4.19      6.25      (.34)     2.51      1.96      4.79     (1.95)     2.78      1.97        (.01)
- ----------------------------------------------------------------------------------------------------------------------------
DISTRIBUTIONS
 Dividends from Net
   Investment
   Income.............     (.34)     (.30)     (.28)     (.23)     (.25)     (.25)     (.33)     (.23)     (.20)         --
 Distributions from
   Realized Capital
   Gains..............    (1.72)     (.40)     (.29)     (.20)     (.18)     (.20)     (.16)     (.23)     (.16)         --
                          -----     -----     -----    ------    ------    ------    ------    ------    ------    --------
   TOTAL
     DISTRIBUTIONS....    (2.06)     (.70)     (.57)     (.43)     (.43)     (.45)     (.49)     (.46)     (.36)         --
- ----------------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, END
 OF PERIOD............   $26.20    $24.07    $18.52    $19.43    $17.35    $15.82    $11.48    $13.92    $11.60       $9.99
============================================================================================================================
TOTAL RETURN*.........    17.65%    33.80%    (1.76)%   14.49%    12.47%    41.85%   (14.05)%   24.10%    19.75%      (0.10)%
============================================================================================================================
RATIOS/SUPPLEMENTAL
 DATA
Net Assets, End of
 Period (Millions)....   $2,099    $1,523      $967      $928      $585      $372      $179      $147       $35          $5
Ratio of Total
 Expenses to Average
 Net Assets...........     0.25%     0.25%     0.20%     0.20%     0.20%     0.19%     0.23%     0.23%     0.24%          0%
Ratio of Net
 Investment Income to
 Average Net Assets...     1.42%     1.51%     1.51%     1.48%     1.73%     2.14%     2.68%     2.92%     2.90%          0%
Portfolio Turnover
 Rate.................       22%       15%       19%       13%        9%       11%        9%       14%       26%          3%
Average Commission
 Rate Paid............   $.0235       N/A       N/A       N/A       N/A       N/A       N/A       N/A       N/A         N/A
*Total return figures do not reflect applicable transaction fees on purchases or the annual account maintenance fee of $10.
+ Commencement of Operations.
</TABLE>
    
 
   
<TABLE>
<CAPTION>
                                                                -------------------------------------------------------------
                                                                   TOTAL STOCK MARKET PORTFOLIO -- PORTFOLIO SHARES**
                                                                -------------------------------------------------------------
                                                                                                                   MARCH 16+,
                                                                            YEAR ENDED DEC. 31,                     1992, TO
                                                                -------------------------------------------         DEC. 31,
                                                                1996         1995         1994         1993           1992
<S>                                                            <C>          <C>          <C>          <C>          <C>     
- -----------------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, BEGINNING OF PERIOD........................   $15.04       $11.37       $11.69       $10.84         $10.00
                                                                -----        -----        -----        -----        -------
INVESTMENT OPERATIONS
 Net Investment Income......................................      .29          .29          .27          .26            .23
 Net Realized and Unrealized Gain (Loss) on Investments.....     2.84         3.75         (.29)         .88            .84
                                                                -----        -----        -----        -----        -------
   TOTAL FROM INVESTMENT OPERATIONS.........................     3.13         4.04         (.02)        1.14           1.07
- -----------------------------------------------------------------------------------------------------------------------------
DISTRIBUTIONS
 Dividends from Net Investment Income.......................     (.29)        (.28)        (.27)        (.26)          (.23)
 Distributions from Realized Capital Gains..................     (.11)        (.09)        (.03)        (.03)            --
                                                                -----        -----        -----        -----        -------
   TOTAL DISTRIBUTIONS......................................     (.40)        (.37)        (.30)        (.29)          (.23)
- -----------------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, END OF PERIOD..............................   $17.77       $15.04       $11.37       $11.69         $10.84
=============================================================================================================================
TOTAL RETURN................................................    20.96%       35.79%       (0.17)%      10.62%         10.41%
=============================================================================================================================
RATIOS/SUPPLEMENTAL DATA
Net Assets, End of Period (Millions)........................   $3,531       $1,571         $786         $512           $275
Ratio of Total Expenses to Average Net Assets...............     0.22%        0.25%        0.20%        0.20%          0.21%*
Ratio of Net Investment Income to Average Net Assets........     1.86%        2.14%        2.35%        2.31%          2.42%*
Portfolio Turnover Rate.....................................        3%           3%           2%           1%             3%
Average Commission Rate Paid................................   $.0216          N/A          N/A          N/A            N/A
   * Annualized.
  ** Total return figures do not reflect the .25% transaction fee on purchases through 1995 or the annual account maintenance
     fee of $10. Subscription period for the Portfolio was from March 16, 1992, to April 26, 1992, during which time all
     assets were held in money market instruments. Performance measurement begins on April 27, 1992.
   + Commencement of operations.
</TABLE>
    
 
                                        8
<PAGE>   12
 
   
<TABLE>
<CAPTION>
                                    ----------------------------------------------------------------------------------------
                                                     SMALL CAPITALIZATION STOCK PORTFOLIO -- PORTFOLIO SHARES(1)
                                    ----------------------------------------------------------------------------------------
                                      YEAR ENDED                        OCT. 1,
                                     DECEMBER 31,        FEB. 1 TO      1993 TO              YEAR ENDED SEPTEMBER 30,
                                    --------------        DEC. 31,      JAN. 31,     ----------------------------------------
                                    1996       1995        1994**        1994**       1993       1992       1991      1990(2)
<S>                                <C>        <C>        <C>            <C>          <C>        <C>        <C>        <C>         
- -----------------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, BEGINNING OF
 PERIOD........................... $18.61     $14.99       $16.24        $16.23      $12.63     $12.03      $8.55     $11.88
                                   ------     ------     --------       -------      ------     ------     ------     -------
INVESTMENT OPERATIONS
 Net Investment Income (Loss).....    .26        .24          .20           .05         .20        .19        .20        .17
 Net Realized and Unrealized Gain
   (Loss) on Investments..........   3.07       4.06         (.86)          .96        3.73        .88       3.60      (3.46) 
                                    -----     ------     --------       -------      ------     ------     ------     -------
   TOTAL FROM INVESTMENT
     OPERATIONS...................   3.33       4.30         (.66)         1.01        3.93       1.07       3.80      (3.29) 
- -----------------------------------------------------------------------------------------------------------------------------
DISTRIBUTIONS
 Dividends from Net Investment
   Income.........................   (.27)      (.23)        (.22)         (.18)       (.18)      (.18)      (.18)      (.04) 
 Distributions from Realized
   Capital Gains..................  (1.44)      (.45)        (.37)         (.82)       (.15)      (.29)      (.14)        --
                                    -----     ------     --------       -------      ------     ------     ------     -------
   TOTAL DISTRIBUTIONS............  (1.71)      (.68)        (.59)        (1.00)       (.33)      (.47)      (.32)      (.04) 
- -----------------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, END OF PERIOD.... $20.23     $18.61       $14.99        $16.24      $16.23     $12.63     $12.03      $8.55
=============================================================================================================================
TOTAL RETURN++....................  18.12%     28.74%       (4.00)%        6.65%      31.60%      9.34%     45.91%    (27.73)% 
=============================================================================================================================
RATIOS/SUPPLEMENTAL DATA
Net Assets, End of Period
 (Millions)....................... $1,713       $971         $605          $533        $432       $202       $111        $40
Ratio of Total Expenses to Average
 Net Assets.......................   0.25%      0.25%        0.17%*        0.18%*      0.18%      0.18%      0.21%      0.31% 
Ratio of Net Investment Income
 (Loss) to
 Average Net Assets...............   1.51%      1.58%        1.50%*        1.16%*      1.47%      1.65%      2.11%      1.91% 
Portfolio Turnover Rate...........     28%        28%          25%            5%         26%        26%        33%        40% 
Average Commission Rate Paid...... $.0245        N/A          N/A           N/A         N/A        N/A        N/A        N/A
</TABLE>
    
 
   
<TABLE>
<CAPTION>
                                                            -----------------------------------------------------------
                                                            SMALL CAPITALIZATION STOCK PORTFOLIO -- PORTFOLIO SHARES(1)
                                                            -----------------------------------------------------------
                                                                                            YEAR ENDED SEPTEMBER 30,
                                                                                     ----------------------------------
                                                                                     1989+          1988          1987
<S>                                                                                  <C>           <C>           <C>       
- -----------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, BEGINNING OF PERIOD..............................................   $11.96        $15.73        $13.24
                                                                                     ------        ------        ------
INVESTMENT OPERATIONS
 Net Investment Income (Loss).....................................................      .10           .03          (.04)
 Net Realized and Unrealized Gain (Loss) on Investments...........................     2.13         (2.59)         4.42
                                                                                     ------        ------        ------
   TOTAL FROM INVESTMENT OPERATIONS...............................................     2.23         (2.56)         4.38
- -----------------------------------------------------------------------------------------------------------------------
DISTRIBUTIONS
 Dividends from Net Investment Income.............................................     (.14)           --            --
 Distributions from Realized Capital Gains........................................    (2.17)        (1.21)        (1.89)
                                                                                     ------        ------        ------
   TOTAL DISTRIBUTIONS............................................................    (2.31)        (1.21)        (1.89)
- -----------------------------------------------------------------------------------------------------------------------
NET ASSET VALUE, END OF PERIOD....................................................   $11.88        $11.96        $15.73
=======================================================================================================================
TOTAL RETURN++....................................................................    18.83%       (14.30)%       38.02%
=======================================================================================================================
RATIOS/SUPPLEMENTAL DATA
Net Assets, End of Period (Millions)..............................................      $20           $27           $35
Ratio of Total Expenses to Average Net Assets.....................................     1.00%         0.95%         0.92%
Ratio of Net Investment Income (Loss) to Average Net Assets.......................      .65%          .24%         (.25)%
Portfolio Turnover Rate...........................................................      160%           68%           92%
Average Commission Rate Paid......................................................      N/A           N/A           N/A
   * Annualized.
  ** Unaudited.
 (1) Results prior to January 31, 1994, are for the former Vanguard Small Capitalization Stock Fund.
 (2) Adjusted to reflect a 3-for-1 stock split as of February 3, 1990.
   + Prior to September 11, 1989, Schroder Capital Management International provided investment advisory services to the
     Fund. Effective September 11, 1989, The Vanguard Group, Inc. began providing investment advisory services to the Fund on
     an at-cost basis.
  ++ Total return figures do not reflect the annual account maintenance fees of $10 or applicable portfolio transaction fees.
</TABLE>
    
 
- --------------------------------------------------------------------------------
 
                                        9
<PAGE>   13
 
   
YIELD AND TOTAL
RETURN                  From time to time each share class of the Funds may
                        advertise its yield and total return. Both yield and
                        total return figures are based on historical earnings
                        and are not intended to indicate future performance. The
                        "total return" of a class of shares of a fund refers to
                        the average annual compounded rates of return over one-,
                        five- and ten-year periods or for the life of the class
                        of shares of the fund (as stated in the advertisement)
                        that would equate an initial amount invested at the
                        beginning of a stated period to the ending redeemable
                        value of the investment, assuming the reinvestment of
                        all dividend and capital gains distributions. Any
                        advertised total returns for Vanguard Index
                        Trust -- Extended Market or Small Capitalization Stock
                        Portfolios will take into account the 0.5% transaction
                        fee on share purchases. The investment results of each
                        class of shares of the Funds will vary, due to their
                        different expenses.
    
 
   
                        In accordance with industry guidelines set forth by the
                        U.S. Securities and Exchange Commission, the Funds
                        calculate the "30-day yield" of each class of shares by
                        dividing the net investment income per share earned by
                        that class during a 30-day period by the net asset value
                        per share on the last day of the period. Net investment
                        income includes interest and dividend income per share
                        earned by the class of shares; it is net of all expenses
                        and all recurring and nonrecurring charges that have
                        been applied to all shareholder accounts of that class.
                        The yield calculation assumes that net investment income
                        per share earned by a class over 30 days is compounded
                        monthly for six months and then annualized. Methods used
                        to calculate advertised yields are standardized for all
                        stock and bond mutual funds. However, these methods
                        differ from the accounting methods used by the Funds to
                        maintain their books and records, and so the advertised
                        30-day yield may not fully reflect the income paid to an
                        investor's account or the yield reported in the Funds
                        reports to shareholders.
    
- --------------------------------------------------------------------------------
 
INVESTMENT
OBJECTIVES              The Funds are open-end, diversified investment companies
                        designed as "index" funds.
  ------------------------------------------------------------------------------
 
   
VANGUARD
INSTITUTIONAL
INDEX FUND
SEEKS TO MATCH
THE INVESTMENT
PERFORMANCE OF THE
S&P 500 INDEX
                        Vanguard Institutional Index Fund is an open-end
                        diversified investment company designed as an "index"
                        fund. It seeks to replicate the aggregate price and
                        yield performance, before Fund expenses, of the Standard
                        & Poor's 500 Composite Stock Price Index (the "S&P 500
                        Index"), an unmanaged index that emphasizes
                        large-capitalization companies. The correlation between
                        the performance of this Fund and the S&P 500 Index is
                        expected to be 0.95 or higher. A correlation of 1.00
                        would indicate perfect correlation. There is no
                        assurance that the Fund will achieve its stated
                        objective.
    
 
                        Vanguard Institutional Index Fund is neither sponsored
                        by or affiliated with Standard & Poor's Corporation.
 
                        The investment objective is fundamental and so cannot be
                        changed without the approval of a majority of the Fund's
                        shareholders.
  ------------------------------------------------------------------------------
 
                                       10
<PAGE>   14
 
   
VANGUARD
INDEX TRUST
THE EXTENDED
MARKET, TOTAL STOCK
MARKET AND SMALL
CAPITALIZATION STOCK
PORTFOLIOS EACH
SEEKS TO MATCH
THE INVESTMENT
PERFORMANCE OF A
PARTICULAR STOCK
MARKET INDEX            Vanguard Index Trust consists of six portfolios, each of
                        which seeks to provide investment results that
                        correspond to a particular stock market index. The
                        correlation between the performance of each portfolio
                        and its respective index is expected to be at least
                        0.95. Three of the portfolios (or "Funds") are offered
                        through this prospectus: Extended Market, Total Stock
                        Market and Small Capitalization Stock Portfolios. Each
                        of these three Funds attempt to replicate the investment
                        performance of broad market indexes.
    
 
                        - The EXTENDED MARKET PORTFOLIO seeks to replicate the
                          aggregate price and yield performance of the Wilshire
                          4500 Index, an index which consists of more than 6,500
                          medium- and small-capitalization companies that are
                          not included in the S&P 500 Index.
 
                        - The TOTAL STOCK MARKET PORTFOLIO seeks to replicate
                          the aggregate price and yield performance of the
                          Wilshire 5000 Index, an index which consists of all
                          U.S. stocks that trade on a regular basis on either
                          the New York or American Stock Exchange or the NASDAQ
                          over-the-counter market. These stocks include the
                          large-capitalization companies of the S&P 500 Index,
                          with the exception of Royal Dutch and Unilever, N.V.,
                          which trade on the New York Stock Exchange as ADR's,
                          as well as the medium- and small-capitalization
                          companies of the Wilshire 4500 Index.
 
                        - The SMALL CAPITALIZATION STOCK PORTFOLIO seeks to
                          replicate the aggregate price and yield performance of
                          the Russell 2000 Small Stock Index (the "Russell
                          2000"), a broadly diversified small-capitalization
                          stock index consisting of approximately 2,000 common
                          stocks.
 
                        There is no assurance that the Portfolios will achieve
                        their stated objectives.
- --------------------------------------------------------------------------------
 
INVESTMENT
POLICIES
THE FUNDS USE
A "PASSIVE"
INVESTMENT APPROACH     The Funds are not managed according to traditional
                        methods of "active" investment management, which involve
                        the buying and selling of securities based upon
                        economic, financial and market analysis and investment
                        judgment. Instead, the Funds utilizing a "passive" or
                        indexing investment approach, attempt to approximate the
                        investment performance of their respective indexes
                        through statistical procedures. The Funds are managed
                        without regard to tax ramifications.
 
                        The Funds are responsible for voting the shares of all
                        securities they hold.
 
   
                        The investment policies of the Funds are not fundamental
                        and so may be changed by the Boards of Trustees without
                        shareholder approval. However, shareholders would be
                        notified prior to a material change.
    
  ------------------------------------------------------------------------------
 
   
VANGUARD
INSTITUTIONAL
INDEX FUND              Vanguard Institutional Index Fund attempts to remain
                        fully invested in common stocks. Under normal
                        circumstances, this Fund will invest at least 95% of its
                        assets in the common stocks of the S&P 500 Index and
                        futures contracts and options. This Fund may invest in
                        certain short-
    
 
                                       11
<PAGE>   15
 
                        term fixed-income securities as cash reserves, although
                        cash or cash equivalents are normally expected to
                        represent less than 1% of its assets. Vanguard
                        Institutional Index Fund may also invest up to 20% of
                        its assets in stock futures contracts and options in
                        order to invest uncommitted cash balances, to maintain
                        liquidity to meet shareholder redemptions, or to
                        minimize trading costs. The Fund will not invest in cash
                        reserves, futures contracts or options as part of a
                        temporary defensive strategy, such as lowering its
                        investment in common stocks to protect against potential
                        stock market declines. Nor may the Fund use futures
                        contracts or options to leverage its net assets in an
                        attempt to speculate on potential stock market gains.
                        See "Implementation of Policies" for a description of
                        these and other investment practices of the Fund.
  ------------------------------------------------------------------------------
 
VANGUARD
INDEX TRUST             The Extended Market, Total Stock Market, and Small
                        Capitalization Stock Portfolios invest in statistically
                        selected samples of the stocks included in each of their
                        respective indexes. This sampling technique is expected
                        to enable each of these Funds to track the price
                        movements of its respective index, while minimizing
                        brokerage, custodial, and accounting costs.
   
EXTENDED MARKET
PORTFOLIO
                        The EXTENDED MARKET PORTFOLIO invests in a statistically
                        selected sample of the more than 6,800 stocks included
                        in the Wilshire 4500 Index. Typically, this Fund invests
                        in approximately 2,000 stocks. Stocks are selected for
                        inclusion in the Fund based primarily on their
                        contribution to the Fund's market capitalization,
                        industry weightings and other fundamental
                        characteristics, such as price-earnings ratios, dividend
                        yields, price-to-book ratios and financial leverage. The
                        stocks held by this Fund are weighted to make the Fund's
                        aggregate investment characteristics similar to those of
                        the Wilshire 4500 Index as a whole.
    
 
   
TOTAL STOCK
MARKET PORTFOLIO
                        The TOTAL STOCK MARKET PORTFOLIO invests in a
                        statistically selected sample of the more than 7,300
                        stocks included in the Wilshire 5000 Index. Typically,
                        this Fund invests in approximately 2,700 stocks. Stocks
                        are selected for inclusion in the Fund based primarily
                        on their contribution to the Fund's market
                        capitalization, industry weightings and other
                        fundamental characteristics such as price-earnings
                        ratios, dividend yields, price-to-book ratios and
                        financial leverage. The stocks held by this Fund are
                        weighted to make the Fund's aggregate investment
                        characteristics similar to those of the Wilshire 5000
                        Index.
    
 
   
SMALL
CAPITALIZATION
STOCK PORTFOLIO
                        The SMALL CAPITALIZATION STOCK PORTFOLIO invests in a
                        statistically selected sample of the approximately 1,900
                        stocks included in the Russell 2000 Index. Typically,
                        this Fund invests in approximately 1,500 stocks. Stocks
                        are selected for inclusion in the Fund based on their
                        contribution to the Fund's market capitalization,
                        industry weightings and other fundamental
                        characteristics, such as price-earnings ratios, dividend
                        yields, price-to-book ratios and financial leverage. The
                        stocks held by the Fund are weighted to make the Fund's
                        aggregate investment characteristics similar to those of
                        the Russell 2000 Index as a whole.
    
 
                                       12
<PAGE>   16
 
                        As with Vanguard Institutional Index Fund, these Funds
                        each attempt to remain fully invested in common stocks.
                        Under normal circumstances each Fund will invest at
                        least 95% of its assets in the common stocks of its
                        respective index and futures contracts and options. Each
                        Fund may invest in certain short-term fixed income
                        securities as cash reserves, although cash or cash
                        equivalents are normally expected to represent less than
                        1% of each Fund's assets. Each Fund may also invest up
                        to 20% of its assets in stock futures contracts and
                        options in order to invest uncommitted cash balances, to
                        maintain liquidity to meet shareholder redemptions, or
                        to minimize trading costs. The Funds will not invest in
                        cash reserves, futures contracts or options as part of a
                        temporary defensive strategy, such as lowering a Fund's
                        investment in common stocks to protect against potential
                        stock market declines. The Funds intend to remain fully
                        invested, to the extent practicable, in a pool of
                        securities which will duplicate the investment
                        characteristics of their respective indexes. See
                        "Implementation of Policies" for a description of these
                        and other investment practices of the Funds.
- --------------------------------------------------------------------------------
 
INVESTMENT
RISKS
THE FUNDS ARE
SUBJECT TO MARKET RISK  As mutual funds investing primarily in common stocks,
                        the Funds are subject to MARKET RISK -- i.e., the
                        possibility that common stock prices will decline over
                        short or even extended periods. Both U.S. and foreign
                        stock markets tend to be cyclical, with periods when
                        stock prices generally rise and periods when prices
                        generally decline.
 
                        Common stocks, as measured by the S&P 500 Index, have
                        provided annual total returns (capital appreciation plus
                        dividend income), averaging +10.8% for all 10-year
                        periods from 1926 to 1996. Average return may not be
                        useful for forecasting future returns in any particular
                        period, as stock returns are quite volatile from year to
                        year.
  ------------------------------------------------------------------------------
 
VANGUARD
INDEX TRUST
THE EXTENDED MARKET,
TOTAL STOCK MARKET
AND SMALL
CAPITALIZATION STOCK
PORTFOLIOS MAY EXHIBIT
GREATER VOLATILITY
RELATIVE TO THE
S&P 500 INDEX
                        Historically, medium- and small-capitalization stocks
                        have been more volatile in price than the
                        larger-capitalization stocks included in the S&P 500
                        Index. Among the reasons for the greater price
                        volatility of these securities are the less certain
                        growth prospects of smaller firms, the lower degree of
                        liquidity in the markets for such stocks, and the
                        greater sensitivity of medium- and small-size companies
                        to changing economic conditions. Besides exhibiting
                        greater volatility, medium- and small-size company
                        stocks may, to a degree, fluctuate independently of
                        larger company stocks. Medium- and small-size company
                        stocks may decline in price as large company stocks
                        rise, or rise in price as large company stocks decline.
                        Medium- and small-size company stocks constitute the
                        investments of the Extended Market Portfolio, while the
                        Small Capitalization Stock Portfolio is composed
                        primarily of small-size company stocks. Investors in
                        these Funds should therefore expect that the Extended
                        Market and Small Capitalization Stock Portfolios will be
                        more volatile than, and may fluctuate independently of,
                        the S&P 500 Index.
 
                        Similarly, medium- and small-size company stocks
                        constituted approximately 30% of the net assets of the
                        Total Stock Market Portfolio on
 
                                       13
<PAGE>   17
 
                        December 31, 1996. Investors in this Fund should
                        therefore also anticipate somewhat greater price
                        volatility in the Total Stock Market Portfolio relative
                        to the S&P 500 Index.
- --------------------------------------------------------------------------------
 
WHO SHOULD
INVEST                  Each Fund is designed for investors seeking to replicate
                        the total return of a different broad market index. The
                        Funds offer investors the advantages of a "passive"
                        approach to investing. These include low investment
                        costs, exceptional diversification among a wide range of
                        stocks, minimal portfolio turnover, and relative
                        predictability. Unlike other mutual funds, which
                        generally attempt to "beat" market averages with often
                        unpredictable results, the Funds seek to "match" the
                        performance of their underlying indexes and thus are
                        expected to provide a highly predictable return relative
                        to these benchmarks.
 
                        However, shareholders should expect to be fully exposed
                        to the market risks inherent in investing in stocks. As
                        the prices of stocks may be volatile, only investors
                        able to tolerate short-term, possibly substantial
                        fluctuations in the value of their investment, brought
                        about by generally declining stock prices, should
                        contemplate an investment in the Funds.
 
                        Investors may wish to reduce the potential risk of
                        investing in the Funds by purchasing shares on a
                        regular, periodic basis (dollar-cost averaging) rather
                        than making an investment in one lump sum.
 
                        The Funds are intended to be long-term investment
                        vehicles and are not designed to provide investors with
                        a means of speculating on short-term market movements.
                        Investors who engage in excessive account activity
                        generate additional costs which are borne by all
                        shareholders. In order to minimize such costs, the Funds
                        have adopted the following policies. Each reserves the
                        right to reject any purchase request (including exchange
                        purchases from other Vanguard portfolios) that is
                        reasonably deemed to be disruptive to efficient
                        portfolio management, either because of the timing of
                        the investment or previous excessive trading by the
                        investor. Additionally, the Funds have adopted exchange
                        privilege limitations as described in the section
                        "Exchange Privilege Limitations." Finally, the Funds
                        reserve the right to suspend the offering of their
                        shares.
 
                        Investors should not consider an investment in any one
                        Fund a complete investment program, but should maintain
                        holdings of securities with different risk
                        characteristics -- including common stocks, bonds and
                        money market instruments.
- --------------------------------------------------------------------------------
 
IMPLEMENTATION
OF POLICIES             The Funds follow a variety of investment practices in an
                        effort to duplicate the total return of their respective
                        indexes.
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VANGUARD
INSTITUTIONAL
INDEX FUND              Vanguard Institutional Index Fund attempts to duplicate
                        the investment results of the S&P 500 Index by holding
                        all 500 stocks in approximately the same proportions as
                        they are represented in the Index. This indexing
                        technique is known as "complete replication."
 
                                       14
<PAGE>   18
 
                        The S&P 500 Index is composed of 500 common stocks,
                        which are chosen by Standard & Poor's Corporation on a
                        statistical basis to be included in the Index. The
                        inclusion of a stock in the S&P 500 Index in no way
                        implies that Standard & Poor's Corporation believes the
                        stock to be an attractive investment. The 500
                        securities, most of which trade on the New York Stock
                        Exchange, represented, as of December 31, 1996,
                        approximately 70% of the market value of all U.S. common
                        stocks. Each stock in the S&P 500 Index is weighted by
                        its market value.
 
                        Because of the market-value weighting, the 50 largest
                        companies in the S&P 500 Index currently account for
                        approximately 47% of the Index. Typically, companies
                        included in the S&P 500 Index are the largest and most
                        dominant firms in their respective industries. As of
                        December 31, 1996, the five largest companies in the
                        Index were: General Electric (2.9%), Coca Cola (2.3%),
                        Exxon Corporation (2.2%), Intel Corporation (1.9%) and
                        Microsoft Corporation (1.7%). The largest industry
                        categories were: banks (7.7%), telephone companies
                        (6.6%), pharmaceutical companies (6.4%), international
                        oil companies (5.8%) and computer companies (4.6%).
 
                        Vanguard Institutional Index Fund is not sponsored,
                        endorsed, sold or promoted by Standard & Poor's
                        Corporation ("S&P"). S&P makes no representation or
                        warranty, implied or express, to the purchasers of the
                        Fund or any member of the public regarding the
                        advisability of investing in index funds or the ability
                        of the S&P 500 Index to track general stock market
                        performance. S&P does not guarantee the accuracy and/or
                        the completeness of the S&P 500 Index or any data
                        included therein.
 
   
                        THE S&P MAKES NO WARRANTY, EXPRESS OR IMPLIED, AS TO
                        RESULTS TO BE OBTAINED BY THE FUND, OWNERS OF THE FUND,
                        OR ANY PERSON OR ENTITY FROM THE USE OF THE S&P 500 OR
                        ANY DATA INCLUDED THEREIN. S&P MAKES NO EXPRESS OR
                        IMPLIED WARRANTIES, AND HEREBY EXPRESSLY DISCLAIMS ALL
                        SUCH WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
                        PARTICULAR PURPOSE FOR USE WITH RESPECT TO THE S&P 500
                        OR ANY DATA INCLUDED THEREIN.
    
 
                        S&P's only relationship to Vanguard Institutional Index
                        Fund is the licensing of the S&P marks and the S&P 500,
                        which is determined, composed and calculated by S&P
                        without regard to the Fund.
 
VANGUARD
INDEX TRUST
THE EXTENDED MARKET
PORTFOLIO INVESTS IN
MEDIUM- AND
SMALL-SIZE COMPANY
STOCKS                  While the S&P 500 Index includes the preponderance of
                        large market capitalization stocks, it excludes most of
                        the medium- and small-size companies which comprise the
                        remaining 30% of the capitalization of the U.S. stock
                        market. The Wilshire 4500 Index consists of all U.S.
                        stocks that are not in the S&P 500 Index and that trade
                        regularly on the New York and American Stock Exchanges
                        as well as in the NASDAQ over-the-counter market. More
                        than 6,500 stocks of medium- and small-capitalization
                        companies are included in the Wilshire 4500 Index.
 
                                       15
<PAGE>   19
 
                        The Extended Market Portfolio will be unable to hold all
                        of the more than 6,500 issues which comprise the
                        Wilshire 4500 Index because of the costs involved and
                        the illiquidity of many of the securities. Instead, this
                        Fund will hold a representative sample of the securities
                        in the Wilshire 4500 Index.
 
THE TOTAL STOCK
MARKET PORTFOLIO
INVESTS IN A SAMPLE OF
ALL U.S. STOCKS         Neither the S&P 500 Index nor the Wilshire 4500 Index
                        independently represents the U.S. stock market as a
                        whole. The Wilshire 5000 Index, which consists of all
                        regularly and publicly traded U.S. stocks, provides a
                        complete proxy for the U.S. stock market. More than
                        7,800 stocks, including large-, medium-, and
                        small-capitalization companies are included in the
                        Wilshire 5000 Index.
 
                        In an effort to replicate the investment performance of
                        the Wilshire 5000 Index, the Total Stock Market
                        Portfolio will invest in approximately 900 of the
                        largest stocks in the index and an additional
                        representative sample of the remaining stocks. As in the
                        case for the Extended Market Portfolio, the high
                        transaction costs and illiquidity of many of the smaller
                        stocks make complete replication of the Wilshire 4500
                        Index's holdings impractical.
 
                        The Extended Market and Total Stock Market Portfolios
                        are not sponsored, endorsed, sold or promoted by
                        Wilshire Associates. Wilshire(R) and Wilshire 5000(R)
                        are registered service marks of Wilshire Associates.
 
THE SMALL
CAPITALIZATION STOCK
PORTFOLIO INVESTS IN
SMALL-SIZE COMPANY
STOCKS                  The Small Capitalization Stock Portfolio attempts to
                        duplicate the investment results of the Russell 2000
                        Index by investing in approximately 1,500 of the 2,000
                        stocks in the Russell 2000 Index. The Russell 2000 Index
                        is composed of approximately 2,000 small-capitalization
                        common stocks. A company's stock market capitalization
                        is the total market value of its floating outstanding
                        shares. As of December 31, 1996, the average stock
                        market capitalization of the Russell 2000 was $660
                        million. As in the case of the Extended Market
                        Portfolio, the high transaction costs and illiquidity of
                        many of the small stocks contained in the Russell 2000
                        Index make complete replication of the holdings
                        impractical.
 
                        The Small Capitalization Stock Portfolio is neither
                        sponsored by nor affiliated with the Frank Russell
                        Company. Frank Russell's only relationship to the
                        Portfolio is the licensing of the use of the Russell
                        2000 Small Stock Index. Frank Russell Company is the
                        owner of the trademarks and copyrights relating to the
                        Russell indexes.
 
THE EXTENDED MARKET,
TOTAL STOCK MARKET
AND SMALL
CAPITALIZATION STOCK
PORTFOLIOS USE
SAMPLING TECHNIQUES     The stocks to be included in the Extended Market, Total
                        Stock Market and Small Capitalization Stock Portfolios
                        will be selected utilizing a statistical sampling
                        technique known as "optimization."
 
                        This process selects stocks for each Fund so that
                        various industry weightings, market capitalizations, and
                        fundamental characteristics (e.g., price-to-book,
                        price-to-earnings, and debt-to-asset ratios, as well as
                        dividend yields) match those of their respective
                        indexes. For instance, if 10% of the capitalization of
                        the Wilshire 5000 consists of utility companies with
                        relatively large market capitalizations, then the Total
                        Stock Market Portfolio's stock holdings are constructed
                        so that approxi-
 
                                       16
<PAGE>   20
 
                        mately 10% of this Fund's stocks represent utilities
                        with relatively large capitalizations.
 
                        This sampling technique is expected to be an effective
                        means of substantially duplicating the income and
                        capital returns of the Extended Market, Total Stock
                        Market and Small Capitalization Stock Portfolios' target
                        benchmarks. Over time, the correlation between the
                        performance of the Extended Market, Total Stock Market
                        and Small Capitalization Stock Portfolios and their
                        respective indexes, the Wilshire 4500 Index, Wilshire
                        5000 Index and Russell 2000 Index, is expected to be at
                        least 0.95. A correlation of 1.00 would indicate perfect
                        correlation, which would be achieved when the net asset
                        value of a Fund, including the value of its dividend and
                        capital gains distributions, increases or decreases in
                        exact proportion to changes in the respective target
                        benchmark.
 
                        Due to the use of the sampling technique, neither the
                        Extended Market Portfolio, Total Stock Market Portfolio
                        nor the Small Capitalization Stock Portfolio is expected
                        to track its benchmark index with the same degree of
                        accuracy as evidenced by the high degree of correlation
                        between Vanguard Institutional Index Fund and its
                        benchmark. However, the principal advantage of this
                        technique is to provide an efficient means to invest in
                        the universe of stocks. In particular, the three Funds
                        using sampling are expected to provide broad
                        diversification, and should operate at low costs due
                        both to their "passive" approach to portfolio management
                        and low portfolio turnover rate.
  ------------------------------------------------------------------------------
 
   
EACH OF THE FUNDS
MAY INVEST IN SHORT-
TERM MONEY MARKET
INSTRUMENTS
                        Although they normally seek to remain substantially
                        fully invested in securities in their respective
                        indexes, the Funds each may invest temporarily in
                        certain short-term money market instruments. Such
                        securities may be used to invest uncommitted cash
                        balances or to maintain liquidity to meet shareholder
                        redemptions. These securities include: obligations of
                        the United States Government and its agencies or
                        instrumentalities; commercial paper, bank certificates
                        of deposit, and bankers' acceptances; and repurchase
                        agreements collateralized by these securities.
    
  ------------------------------------------------------------------------------
 
DERIVATIVE INVESTING    Derivatives are instruments whose values are linked to
                        or derived from an underlying security or index. The
                        most common and conventional types of derivative
                        securities are futures and options.
 
                        The Funds may utilize stock futures contracts, options,
                        warrants, and swap agreements to a limited extent.
                        Specifically, each Fund may enter into futures contracts
                        and options provided that not more than 5% of its assets
                        are required as a margin deposit for futures contracts
                        or options. Furthermore, not more than 20% of each
                        Fund's assets are to be invested in futures and options
                        at any time. Additionally, the Funds' investments in
                        warrants will not exceed more than 5% of each of their
                        assets (2% with respect to warrants not listed on the
                        New York or American Stock Exchanges).
 
                                       17
<PAGE>   21
 
                        The Funds may invest in convertible securities but these
                        investments are not expected to exceed 5% of each of
                        their assets.
 
   
                        The risk of loss associated with futures contracts in
                        some strategies can be substantial due both to the low
                        margin deposits required and the extremely high degree
                        of leverage involved in futures pricing. As a result, a
                        relatively small price movement in a futures contract
                        may result in an immediate and substantial loss or gain.
                        However, the Funds' will not use futures contracts,
                        options, warrants, convertible securities or swap
                        agreements for speculative purposes or to leverage their
                        net assets. Accordingly, the primary risks associated
                        with a Fund's use of these investments are: (i)
                        imperfect correlation between the change in market value
                        of the stocks held by a Fund and the prices of futures
                        contracts and options; and (ii) possible lack of a
                        liquid secondary market for a futures contract and the
                        resulting inability to close a futures position prior to
                        its maturity date. The risk of imperfect correlation
                        will be minimized by investing only in those contracts
                        whose behavior is expected to resemble that of the
                        Fund's underlying securities. The risk that a Fund will
                        be unable to close out a futures position will be
                        minimized by entering into such transactions on a
                        national exchange with an active and liquid secondary
                        market. However, options, warrants, convertible
                        securities and swap agreements purchased or sold
                        over-the-counter may be less liquid than exchange traded
                        securities. Illiquid securities, in general, may not
                        represent more than 15% of the net assets of any Fund.
    
 
                        Swap agreements are contracts between parties in which
                        one party agrees to make payments to the other party
                        based on the change in market value of a specified index
                        or asset. In return, the other party agrees to make
                        payments to the first party based on the return of a
                        different specified index or asset. Although swap
                        agreements entail the risk that a party will default on
                        its payment obligations thereunder, the Funds will
                        minimize this risk by entering into agreements that mark
                        to market no less frequently than quarterly. Swap
                        agreements also bear the risk that a Fund will not be
                        able to meet its obligation to the counterparty. This
                        risk will be mitigated by a Fund's investment in the
                        specific asset for which it is obligated to pay a
                        return.
  ------------------------------------------------------------------------------
 
SECURITY LENDING
                        The Funds may lend their investment securities to
                        qualified institutional investors for either short-term
                        or long-term purposes of realizing additional income.
                        Loans of securities by the Funds will be collateralized
                        by cash, letters of credit, or securities issued or
                        guaranteed by the U.S. Government or its agencies. The
                        collateral will equal at least 100% of the current
                        market value of the loaned securities. Each Fund will
                        limit such loans so that they will not exceed 33 1/3 of
                        the value of its securities.
  ------------------------------------------------------------------------------
 
                                       18
<PAGE>   22
 
PORTFOLIO TURNOVER FOR
THE FUNDS IS EXPECTED
TO BE LOW               Although they generally seek to invest for the long
                        term, the Funds retain the right to sell securities
                        irrespective of how long they have been held. It is
                        anticipated that the annual portfolio turnover of each
                        Fund will not exceed 50%. A turnover rate of 50% would
                        occur, for example, if one half of the securities of a
                        Fund were replaced within one year.
- --------------------------------------------------------------------------------
 
   
INVESTMENT
LIMITATIONS
THE FUNDS HAVE
ADOPTED CERTAIN
FUNDAMENTAL
LIMITATIONS             Each Fund has adopted certain limitations on its
                        investment practices. Specifically, each Fund will not:
    
 
                        (a)  invest more than 25% of its total assets in any one
                             industry;
 
                        (b)  borrow money, except that the Fund may borrow from
                             banks (or through reverse repurchase agreements),
                             for temporary or emergency (not leveraging)
                             purposes, including the meeting of redemption
                             requests which might otherwise require the untimely
                             disposition of securities, in an amount not
                             exceeding 15% of the value of the Fund's net assets
                             (including the amount borrowed and the value of any
                             outstanding reverse repurchase agreements) at the
                             time the borrowing is made. Whenever borrowings
                             exceed 5% of the value of a Fund's net assets, the
                             Fund will not make any additional investments.
 
   
                        (c)  with respect to 75% of its assets, (i) purchase
                             securities of any issuer (except obligations of the
                             U.S. Government and its instrumentalities) if, as a
                             result, more than 5% of the value of the Fund's
                             assets would be invested in the securities of such
                             issuer; or (ii) purchase more than 10% of the
                             outstanding voting securities of any issuer.
    
 
                        These investment limitations are considered at the time
                        investment securities are purchased. The limitations
                        described here and in the Statements of Additional
                        Information relating to the Funds may be changed only
                        with the approval of a majority of the shareholders of
                        the applicable Fund.
- --------------------------------------------------------------------------------
 
   
MANAGEMENT AND
INVESTMENT
ADVISORY SERVICES       The Funds receive all of their essential
                        services -- including management, administrative,
                        shareholder, investment advisory, distribution and
                        marketing -- from The Vanguard Group, Inc. ("Vanguard").
                        Vanguard Institutional Index Fund employs Vanguard, and
                        pays its fees, to provide these services. As part of
                        Vanguard Index Trust, the Extended Market, Total Stock
                        Market and Small Capitalization Stock Portfolios receive
                        services from Vanguard on an at-cost basis.
    
  ------------------------------------------------------------------------------
 
VANGUARD ADMINISTERS
AND PROVIDES ADVISORY
SERVICES TO THE FUNDS
                        Vanguard Index Trust is a member of The Vanguard Group
                        of Investment Companies, a family of more than 30
                        investment companies with more than 90 distinct
                        investment portfolios and total assets in excess of $250
                        billion. Through their jointly-owned subsidiary,
                        Vanguard, Vanguard Index Trust and the other funds in
                        the Group obtain virtually all of their corporate
                        management, administrative, shareholder accounting and
                        distribution services. Vanguard also provides investment
                        advisory services to Vanguard Index Trust and to certain
                        Vanguard funds on an at-cost basis.
 
                                       19
<PAGE>   23
 
   
                        As a result of Vanguard's unique corporate structure,
                        the Vanguard funds have costs substantially lower than
                        those of most competing mutual funds. In 1996, the
                        average expense ratio (annual costs including advisory
                        fees divided by total net assets) for the Vanguard funds
                        amounted to approximately .29% compared to an average of
                        1.22% for the mutual fund industry (data provided by
                        Lipper Analytical Services). Each fund that is a member
                        of The Vanguard Group of Investment Companies pays its
                        share of Vanguard's net expenses, which are allocated
                        among the funds under methods approved by the Board of
                        Directors (Trustees) of each fund. In addition, each
                        fund bears its own direct expenses, such as legal,
                        auditing and custodian fees.
    
  ------------------------------------------------------------------------------
 
VANGUARD PROVIDES
MANAGEMENT AND
ADMINISTRATIVE
PERSONNEL TO
THE FUNDS
                        Vanguard employs a supporting staff of management and
                        administrative personnel to provide the requisite
                        services to the Funds and also furnishes the Funds with
                        the necessary office space, furnishings and equipment.
 
   
                        The Officers of Vanguard Institutional Index Fund and
                        Vanguard Index Trust manage the day-to-day operations of
                        the Funds and are responsible to their respective Boards
                        of Trustees. The Trustees set broad policies for
                        Vanguard Institutional Index Fund and Vanguard Index
                        Trust and choose their Officers. A list of Trustees and
                        Officers and a statement of their present positions and
                        principal occupations during the past five years can be
                        found in the Statements of Additional Information of the
                        Funds.
    
 
   
                        The Funds are not actively managed, but are instead
                        administered by Vanguard Core Management Group using
                        computerized, quantitative techniques.
    
 
                        The Core Management Group manages the investment and
                        reinvestment of the Funds' assets and continuously
                        reviews, supervises, and administers the Funds'
                        investment programs with respect to those assets.
 
   
                        The Core Management Group also provides investment
                        advisory services to several other Vanguard funds,
                        including Vanguard Balanced Index Fund, Vanguard
                        International Equity Index Fund, the Equity Index
                        Portfolio of the Vanguard Variable Insurance Fund, the
                        Growth and Income and Capital Appreciation Portfolios
                        and the equity portion of the Balanced Portfolio of
                        Vanguard Tax-Managed Fund, the Aggressive Growth
                        Portfolio of Vanguard Horizon Fund, the REIT Index of
                        Vanguard Specialized Portfolio, a portion of
                        Vanguard/Morgan Growth Fund, Vanguard/Windsor II and
                        (soon) Vanguard/Explorer Fund as well as to several
                        indexed separate accounts. Total assets under management
                        by the Core Management Group were approximately $57
                        billion as of December 31, 1996.
    
  ------------------------------------------------------------------------------
 
                                       20
<PAGE>   24
 
VANGUARD
INSTITUTIONAL
INDEX FUND PAYS
VANGUARD A
QUARTERLY FEE           Under the terms of a Service and Advisory Agreement with
                        Vanguard, Vanguard pays all of Vanguard Institutional
                        Index Fund's expenses, except for taxes and brokerage
                        commissions. In turn, the Fund pays Vanguard a fee at
                        the end of each fiscal quarter, calculated by applying a
                        quarterly rate, based on the annual percentage rate of
                        .02%, to average daily net assets for the quarter for
                        advisory services, corporate management and
                        administrative services and .005% for the provision of
                        shareholder services with respect to the Institutional
                        Plus Shares and .04% for the provision of shareholder
                        services with respect to the Institutional Shares. The
                        Institutional Plus Shares and the Institutional Shares
                        each bear their own expenses for shareholder services
                        provided by Vanguard.
 
                        Under a previous Service and Advisory Agreement with
                        Vanguard for the 1996 fiscal year, the Fund paid
                        Vanguard an investment advisory and administrative
                        services fee which represented an effective annual rate
                        of .06 of 1% of average (daily) net assets; this
                        reflects the total expenses as a percentage of average
                        net assets of the Institutional Shares of the Fund for
                        the 1996 fiscal year.
  ------------------------------------------------------------------------------
 
   
VANGUARD MANAGES
VANGUARD INDEX TRUST
ON AN AT-COST BASIS
                        Vanguard Index Trust -- Extended Market, Total Stock
                        Market and Small Capitalization Portfolios receive all
                        investment advisory services on an at-cost basis from
                        Vanguard's Core Management Group.
    
 
   
                        For the fiscal year ended December 31, 1996 the total
                        advisory expenses as a percentage of average net assets
                        of the Portfolio Shares of Total Stock Market Portfolio,
                        Extended Market Portfolio and Small Capitalization Stock
                        Portfolio were .22%, .25%, and .25%, respectively.
    
 
                        The Institutional Shares and the Portfolio Shares of
                        these Funds each bear their own expenses relating to
                        marketing and distribution, account maintenance and
                        shareholder services.
- --------------------------------------------------------------------------------
 
   
PORTFOLIO
TRANSACTIONS            In placing portfolio transactions for the Funds,
                        Vanguard's Core Management Group uses its best judgment
                        to choose the broker most capable of providing the
                        brokerage services necessary to obtain the best
                        available price and most favorable execution at the
                        lowest commission rate. The full range and quality of
                        brokerage services available are considered in making
                        these determinations. In those instances where it is
                        reasonably determined that more than one broker can
                        offer the services needed to obtain the best available
                        price and most favorable execution, consideration may be
                        given to those brokers which supply statistical
                        information and provide other services in addition to
                        execution services to the Funds. However, the Core
                        Management Group will not pay higher commissions
                        specifically for the purpose of obtaining research
                        services.
    
- --------------------------------------------------------------------------------
 
                                       21
<PAGE>   25
 
DIVIDENDS, CAPITAL
GAINS AND TAXES         Vanguard Institutional Index Fund and the Total Stock
                        Market Portfolio each distributes substantially all of
                        its ordinary income in the form of quarterly dividends.
                        The Extended Market and Small Capitalization Stock
                        Portfolios each pay annual dividends. Capital gains
                        distributions for each Fund, if any, are made annually.
                        The Funds' dividend and capital gains distributions may
                        be reinvested in additional shares or received in cash.
                        See "Distribution Options."
 
                        Pursuant to the Internal Revenue Code, certain dividend
                        and capital gains distributions declared by each Fund
                        during December, if received by shareholders by January
                        31, are deemed to have been paid by the Fund and
                        received by shareholders on December 31 of the prior
                        year.
 
                        The Funds intend to continue to qualify for taxation as
                        a "regulated investment company" under the Internal
                        Revenue Code so that they will not be subject to federal
                        income tax to the extent its income is distributed to
                        shareholders. Dividends paid by the Funds from net
                        investment income, whether received in cash or
                        reinvested in additional shares, will be taxable to
                        shareholders as ordinary income. For corporate
                        investors, dividends from net investment income will
                        generally qualify in part for the intercorporate
                        dividends-received deduction. However, the portion of
                        the dividends so qualified depends on the aggregate
                        taxable qualifying dividend income received by the Funds
                        from domestic (U.S.) sources.
 
                        Distributions paid by the Funds from long-term capital
                        gains, whether received in cash or reinvested in
                        additional shares, are taxable as long-term capital
                        gains, regardless of the length of time the shares have
                        been owned. Capital gains distributions are made when a
                        Fund realizes net capital gains on sales of portfolio
                        securities during the year. The Funds do not seek to
                        realize any particular amount of capital gains during a
                        year; rather, realized gains are a by-product of
                        portfolio management activities. Consequently, capital
                        gains distributions may be expected to vary considerably
                        from year to year; there will be no capital gains
                        distributions in years when a Fund realizes net capital
                        losses.
 
                        Note that if you elect to receive capital gains
                        distributions in cash, instead of reinvesting them in
                        additional shares, you are in effect reducing the
                        capital at work for you in a Fund. Also, keep in mind
                        that if you purchase shares in a Fund shortly before the
                        record date for a dividend or capital gains
                        distribution, a portion of your investment will be
                        returned to you as a taxable distribution, regardless of
                        whether you are reinvesting your distributions or
                        receiving them in cash.
 
                        The Funds will notify you annually as to the tax status
                        of dividend and capital gains distributions paid by
                        them.
 
                        Dividends and capital gains are calculated and
                        distributed the same way for each class of shares. The
                        amount of any income dividends per share will vary,
                        however, generally due to the differences in shareholder
                        services expenses for Vanguard Institutional Index
                        Fund's separate share classes and differences in
                        marketing and distribution, account maintenance and
                        shareholder services expenses for Vanguard Index Trust's
                        separate share classes.
 
                                       22
<PAGE>   26
 
A CAPITAL GAIN OR LOSS
MAY BE REALIZED UPON
EXCHANGE OR
REDEMPTION              A sale of shares of a Fund is a taxable event, and may
                        result in a capital gain or loss. A capital gain or loss
                        may be realized from an ordinary redemption of shares or
                        an exchange of shares between two mutual funds (or two
                        portfolios of the same fund).
 
                        Dividend distributions, capital gain distributions, and
                        capital gains or losses from redemptions and exchanges
                        may be subject to state and local taxes.
 
                        Each Fund is required to withhold 31% of taxable
                        dividends, capital gains distributions, and redemptions
                        paid to shareholders who have not complied with IRS
                        taxpayer identification regulations. You may avoid this
                        withholding requirement by certifying on your Account
                        Registration Form your proper Social Security or
                        Employer Identification number and by certifying that
                        you are not subject to backup withholding.
 
                        Vanguard Institutional Index Fund and Vanguard Index
                        Trust are organized as Pennsylvania business trusts and,
                        in the opinion of counsel, are not liable for any income
                        or franchise tax in the Commonwealth of Pennsylvania.
                        Vanguard Institutional Index Fund and Vanguard Index
                        Trust will be subject to Pennsylvania county personal
                        property tax (if any) in the county which is the site of
                        its principal office. Shareholders who are Pennsylvania
                        residents generally will not be subject to county
                        personal property taxes.
 
                        The tax discussion set forth above is included for
                        general information only. Prospective investors should
                        consult their own tax advisers concerning the tax
                        consequences of an investment in the Funds.
- --------------------------------------------------------------------------------
 
   
THE SHARE PRICE
OF EACH FUND            The share price or "net asset value" per share of each
                        class of the Funds equals net assets attributable to a
                        class divided by shares outstanding of the class. Net
                        asset value per share is determined once daily at the
                        close of regular trading on the New York Stock Exchange
                        (generally 4:00 p.m. Eastern time).
    
 
   
                        Portfolio securities held by the Funds that are listed
                        on a securities exchange are valued at the last quoted
                        sales price on the day the valuation is made. Price
                        information on listed securities is taken from the
                        exchange where the security is primarily traded.
                        Securities which are listed on an exchange and which are
                        not traded on the valuation date are valued at the mean
                        of the bid and ask prices. For Vanguard Institutional
                        Index Fund, unlisted securities for which market
                        quotations are readily available are valued at the
                        latest quoted bid price. For the Extended Market, Total
                        Stock Market and Small Capitalization Stock Portfolios,
                        unlisted securities for which market quotations are
                        readily available are valued at the mean of the bid and
                        ask prices. Temporary cash investments are valued at
                        amortized cost which approximates market value. For each
                        Fund, securities for which no current quotations are
                        readily available are valued at fair market value as
                        determined in good faith by the Trustees. Securities may
                        be valued on the basis of prices provided by a pricing
                        service when such prices are believed to reflect the
                        fair market value of such securities.
    
 
                                       23
<PAGE>   27
 
                        Each Fund's share price can be found daily in the mutual
                        fund listings of most major newspapers under the heading
                        of Vanguard.
- --------------------------------------------------------------------------------
 
   
GENERAL
INFORMATION             Vanguard Institutional Index Fund and Vanguard Index
                        Trust are Pennsylvania business trusts. Their
                        Declarations of Trust permit the Trustees to issue an
                        unlimited number of shares of beneficial interest with
                        no par value. Their Boards of Trustees have the power to
                        designate one or more classes or series of shares of
                        beneficial interest and to classify or reclassify any
                        unissued shares with respect to such series or classes.
                        Currently, Vanguard Institutional Index Fund is offering
                        shares of one series and Vanguard Index Trust is
                        offering shares of six series.
    
 
   
                        Vanguard Institutional Index Fund offers two distinct
                        classes of shares, the Institutional Shares and the
                        Institutional Plus Shares. The Institutional Shares are
                        available to investors who meet the minimum initial
                        investment of $10 million and may require special
                        employee benefit plan services. The Institutional Plus
                        Shares are available to investors who meet the minimum
                        initial investment of $200 million and do not require
                        special employee benefit plan services. The
                        Institutional Shares and the Institutional Plus Shares
                        each bear their own expenses for shareholder services;
                        the Institutional Shares are subject to higher expenses
                        for such services, which may affect performance.
    
 
   
                        The Extended Market, Total Stock Market and Small
                        Capitalization Stock Portfolios of Vanguard Index Trust
                        each offer two distinct classes of shares, the Portfolio
                        Shares and the Institutional Shares. The Portfolio
                        Shares are available to investors who meet the minimum
                        initial investment of $3,000 ($1,000 for retirement plan
                        accounts) and may require special employee benefit plan
                        services. The Institutional Shares of each Portfolio are
                        available to investors who meet the minimum initial
                        investment of $10 million and generally do not require
                        special employee benefit plan services. The Portfolio
                        Shares and the Institutional Shares each bear their own
                        expenses for marketing and distribution and account
                        maintenance. It is expected that the expenses to be
                        borne by the Portfolio Shares will be higher than the
                        expenses to be borne by the Institutional Shares, which
                        may affect performance.
    
 
                        The shares of Vanguard Institutional Index Fund and each
                        series of Vanguard Index Trust are fully paid and
                        non-assessable; have no preference as to conversion,
                        exchange, dividends, retirement or other features; and
                        have no preemptive rights. Such shares have non-
                        cumulative voting rights, meaning that the holders of
                        more than 50% of the shares voting for the election of
                        Trustees can elect 100% of the Trustees if they so
                        choose.
 
                        Annual meetings of shareholders will not be held except
                        as required by the Investment Company Act of 1940 and
                        other applicable law. An annual meeting will be held to
                        vote on the removal of a Trustee or Trustees of Vanguard
                        Institutional Index Fund or Vanguard Index Trust if
                        requested in writing by the holders of not less than 10%
                        of the outstanding shares of Vanguard Institutional
                        Index Fund or Vanguard Index Trust, as applicable.
 
                                       24
<PAGE>   28
 
                        All securities and cash for Vanguard Institutional Index
                        Fund and the Small Capitalization Stock Portfolio are
                        held by CoreStates Bank, N.A. All securities and cash
                        for the Extended Market and Total Stock Market
                        Portfolios of Vanguard Index Trust are held by State
                        Street Bank and Trust Company. CoreStates Bank, N.A.
                        holds daily cash balances that are used by these two
                        Funds to invest in repurchase agreements or securities
                        acquired in these transactions.
 
                        Vanguard, Valley Forge, PA, serves as the Transfer and
                        Dividend Disbursing Agent for the Funds. Price
                        Waterhouse LLP serves as independent accountants for the
                        Funds and will audit their financial statements
                        annually. Neither Vanguard Institutional Index Fund nor
                        Vanguard Index Trust is involved in any litigation.
- --------------------------------------------------------------------------------
 
                                       25
<PAGE>   29
 
                               SHAREHOLDER GUIDE
 
OPENING AN
ACCOUNT AND
PURCHASING
SHARES                  To open a new account in Vanguard Institutional Index
                        Fund, complete an Account Registration Form and mail it
                        to:
 
   
                                       Vanguard Financial Center
                                       Vanguard Institutional Index Fund
                                       Name of Class
                                       Attn: Institutional Investor Services
                                       P.O. Box 1472
                                       Valley Forge, PA 19482-1472
    
 
VANGUARD
INSTITUTIONAL
INDEX FUND              For express or registered mail, send your registration
                        form to: Vanguard Financial Center, Vanguard
                        Institutional Index Fund, Attn: Institutional Investor
                        Services, 100 Vanguard Boulevard, Malvern, PA 19355.
 
                        Once the account has been opened, Vanguard will assign a
                        Service Representative for future account transactions.
 
                        Because of the risks associated with common stock
                        investments, this Fund is intended to be a long-term
                        investment vehicle and is not designed to provide
                        investors with a means of speculating on short-term
                        stock market movements. Consequently, the Fund reserves
                        the right to reject any specific purchase (or exchange
                        purchase) request. The Fund also reserves the right to
                        suspend the offering of shares for a period of time.
 
   
                        Shares of the Fund generally may be purchased by Federal
                        Funds wire. The minimum initial investment for the Fund
                        is $10 million for the Institutional Shares and $200
                        million for the Institutional Plus Shares. Please
                        contact your Institutional Investor Services
                        Representative at (1-800-523-1036) to notify the Fund of
                        the intended investment and to receive an account
                        number. Wiring instructions are provided below.
    
 
                        Subsequent investments of $5 million or more will be
                        credited to an account on the date of purchase if
                        Vanguard is notified one business day in advance of the
                        intended purchase and a Federal Funds wire is received
                        by 4:00 p.m. (Eastern time) on the date of purchase. See
                        "Trade Date Policy."
 
   
VANGUARD
INDEX TRUST --
EXTENDED
MARKET PORTFOLIO,
TOTAL STOCK MARKET
PORTFOLIO AND
SMALL CAPITALIZATION
STOCK PORTFOLIO         To open an account in Vanguard Index Trust, complete an
                        Account Registration Form and mail it to:
    
 
   
                                       Vanguard Financial Center
                                       Vanguard Index Trust
                                       NAME OF PORTFOLIO -- INSTITUTIONAL SHARES
                                       Attn: Institutional Investor Services
                                       P.O. Box 1472
                                       Valley Forge, PA 19482-1472
    
 
                        For express or registered mail, send your registration
                        form to: Vanguard Financial Center, Vanguard Index
                        Trust, NAME OF PORTFOLIO -- Institutional Shares, Attn:
                        Institutional Investor Services, 100 Vanguard Boulevard,
                        Malvern, PA 19355.
 
                                       26
<PAGE>   30
 
                        Once the account has been opened, Vanguard will assign a
                        Service Representative for future account transactions.
 
                        Because of the risks associated with common stock
                        investments, these Funds are intended to be long-term
                        investment vehicles and are not designed to provide
                        investors with a means of speculating on short-term
                        stock market movements. Consequently, the Funds reserve
                        the right to reject any specific purchase (or exchange
                        purchase) request. The Funds also reserve the right to
                        suspend the offering of shares for a period of time.
 
   
                        Shares of the Funds generally may be purchased by
                        Federal Funds wire. The minimum initial investment for
                        the Institutional Shares of each Vanguard Index Trust
                        Portfolio is $10 million. Please contact your
                        Institutional Investor Services Representative at
                        (1-800-523-1036) to notify the Funds of the intended
                        investment and to receive an account number. Wiring
                        instructions are provided below.
    
 
                        Subsequent investments of $5 million or more will be
                        credited to an account on the date of purchase if
                        Vanguard is notified one business day in advance of the
                        intended purchase and a Federal Funds wire is received
                        by 4:00 p.m. (Eastern time) on the date of purchase. See
                        "Trade Date Policy."
 
   
IMPORTANT NOTE
ON PORTFOLIO
TRANSACTION FEES        The Extended Market Portfolio and the Small
                        Capitalization Stock Portfolio each assess a portfolio
                        transaction fee on purchases of shares equal to 0.5% of
                        the dollar amount invested.
    
 
                        Vanguard Institutional Index Fund and the Total Stock
                        Market Portfolio reserve the right to deduct a portfolio
                        transaction fee, ranging from 0.08% to 0.20%, from
                        purchases of their shares, if such purchase or
                        cumulative purchases are of a size that is reasonably
                        deemed to be disruptive to efficient portfolio
                        management. Please call Vanguard's Institutional
                        Investor Services Department before investing to
                        determine whether a fee will be charged for your share
                        purchases.
 
   
                        Portfolio transaction fees for the Funds apply to
                        initial investments and all subsequent purchases
                        (including purchases made by exchange from another
                        Vanguard fund) but not to reinvested dividend or capital
                        gains distributions. Portfolio transaction fees are
                        deducted automatically from the amount invested, they
                        cannot be paid separately. The portfolio transaction fee
                        will be paid to the Fund to offset transaction costs of
                        buying securities. The fee is not paid to Vanguard and
                        is not a sales charge.
    
 
                        The Extended Market, Total Stock Market and Small
                        Capitalization Portfolios also charge a $10 annual
                        account maintenance fee for Portfolio Shares accounts
                        with balances less than $10,000.
 
                                       27
<PAGE>   31
 
ADDITIONAL
INVESTMENTS
Please contact
your Service
Representative
                        Additional investments may be made at any time by wiring
                        monies to Vanguard. As noted above, investments of $5
                        million or more require prior-day notification to
                        qualify for credit on the date of purchase. To ensure
                        prompt investment, please notify your Institutional
                        Investor Services Representative in advance of the wire.
  ------------------------------------------------------------------------------
 
<TABLE>
<S>                       <C>
PURCHASING BY WIRE        Monies should be wired to:
BEFORE WIRING                               CORESTATES BANK, N.A.
Please contact your                         ABA 031000011
Service Representative                      CORESTATES ACCT NO 0101-9897
                                            ATTN VANGUARD
                                            NAME OF FUND
                                            NAME OF PORTFOLIO
                                            NAME OF CLASS
                                            ACCOUNT NUMBER
                                            ACCOUNT REGISTRATION
                          To ensure proper receipt, please be sure to include in the
                          wiring instructions the complete name of the Fund, the account
                          number Vanguard has assigned you and the eight-digit CoreStates
                          number. NOTE: Federal Funds wire purchase orders will be
                          accepted only when the Funds and Custodian Bank are open for
                          business.
</TABLE>
 
  ------------------------------------------------------------------------------
 
   
PURCHASING BY
EXCHANGE (from a
Vanguard account)       Purchases may also be made by exchange from an existing
                        Vanguard Fund account. However, the Funds reserve the
                        right to refuse any exchange purchase request. Please
                        call your Service Representative.
    
  ------------------------------------------------------------------------------
 
DISTRIBUTION OPTIONS    Dividend and capital gains distributions paid by the
                        Funds will be automatically reinvested in additional
                        shares of the same class of shares of the Fund that you
                        own. A cash dividend option is also available. Please
                        contact your Service Representative for further
                        information.
 
CERTIFICATES            Share certificates will not be issued for any Fund.
 
   
ELECTRONIC PROSPECTUS
DELIVERY                If you would prefer to receive a prospectus for a Fund
                        or any of the Vanguard Funds in an electronic format,
                        please visit Vanguard's web site at www.vanguard.com. If
                        you elect to do so, you may also receive a paper copy of
                        the prospectus, by calling 1-800-523-1036.
    
- --------------------------------------------------------------------------------
 
TRADE DATE POLICY       Investments will be credited on the date of purchase
                        under the following conditions:
 
                        - FOR INVESTMENTS OF $5 MILLION OR MORE: The Fund must
                          be notified of the intended purchase by the close of
                          the New York Stock Exchange, (generally 4:00 p.m.
                          Eastern time) on the prior business day and the
                          Federal Funds wire must be received by Vanguard by the
                          close of the Exchange on the date of purchase.
 
                        - FOR INVESTMENTS OF LESS THAN $5 MILLION: The Fund must
                          be notified of the intended purchase by 10:45 a.m.
                          (Eastern time) on the day of
 
                                       28
<PAGE>   32
 
                         purchase and the Federal Funds wire must be received by
                         the close of the Exchange.
 
                        Generally, if these requirements are not met, an
                        investment will be credited to the account on the
                        business day following receipt of a Federal Funds wire.
 
                        The trade date, the day on which an account is credited,
                        is generally the day on which the Fund receives an
                        investment in the form of Federal Funds. For purchases
                        by Federal Funds wire or by exchange, the Fund is
                        credited immediately with Federal Funds. If a purchase
                        by Federal Funds wire or exchange is received by the
                        close of the New York Stock Exchange, (generally 4:00
                        p.m. Eastern time), the trade date is the day of receipt
                        assuming proper notification has been given, as
                        described above. If a purchase is received after the
                        close of the Exchange, the trade date is the business
                        day following the receipt of the wire or exchange.
- --------------------------------------------------------------------------------
 
SELLING YOUR
SHARES
WIRE PROCEEDS           Any portion of an account may be withdrawn by contacting
                        your Service Representative. The redemption proceeds
                        will be wired to the bank account indicated on the
                        Account Registration Form within five business days
                        following receipt of a request.
 
                        Wire redemptions of less than $5,000 are subject to a $5
                        charge deducted from the principal in your account.
                        There is no charge for wire redemptions of $5,000 or
                        more, or for subsequent dividend wires.
 
                        For our mutual protection, wiring instructions must be
                        on file at Vanguard prior to executing any redemption
                        request. A request to change the bank account associated
                        with the wire redemption feature or a request to wire
                        funds to a bank other than that on file must be received
                        in writing. A signature guarantee of an authorized
                        officer is required if the bank registration is not
                        identical to your account registration.
  ------------------------------------------------------------------------------
 
OTHER REDEMPTION
INFORMATION             Each Fund may suspend the redemption rights or postpone
                        payment at times when the New York Stock Exchange is
                        closed or under any emergency circumstances as
                        determined by the United States Securities and Exchange
                        Commission.
 
                        If the Board of Trustees determines that it would be
                        detrimental to the best interests of a Fund's remaining
                        shareholders to make payment in cash, the Fund may pay
                        redemption proceeds in whole or in part by a
                        distribution in kind of readily marketable securities.
 
   
                        Vanguard Institutional Index Fund, reserves the right to
                        redeem Institutional Shares of any account with a
                        balance of less than $10 million. This action will be
                        taken when the balance of the account falls below $10
                        million due to account redemptions. Reductions in
                        account balances due to market depreciation will not be
                        considered until the account balance declines to less
                        than $5 million. Investors will be provided with 60
                        days' notice before any such action is taken.
    
  ------------------------------------------------------------------------------
 
                                       29
<PAGE>   33
 
   
MANDATORY
CONVERSION TO
INSTITUTIONAL SHARES
OR PORTFOLIO
SHARES                  Vanguard Institutional Index Fund reserves the right to
                        convert an investor's Institutional Plus Shares into the
                        Institutional Shares of the Fund if the investor's
                        account balance falls below $200 million. The Extended
                        Market, Total Stock Market and Small Capitalization
                        Stock Portfolios, each reserves the right to convert an
                        investor's Institutional Shares into the Portfolio
                        Shares of the same Fund if the investor's account
                        balance falls below $10 million. Any such conversion
                        will be preceded by written notice to the investor.
                        There will be no portfolio transaction fee imposed on
                        share class conversions.
    
- --------------------------------------------------------------------------------
 
EXCHANGING YOUR
SHARES                  Shares of the Funds may be exchanged for those of other
                        available Vanguard funds, but only upon prior approval
                        by Vanguard. Exchanges without prior Vanguard
                        authorization are not permitted for the Funds. Contact
                        your Service Representative for further information.
 
   
                        Through December 31, 1997, Portfolio Shares of the
                        Extended Market, Total Stock Market and Small
                        Capitalization Stock Portfolios may be exchanged for the
                        Institutional Shares of the corresponding Fund, provided
                        that the investor satisfies all purchase eligibility
                        requirements for the Institutional Shares.
    
 
                        Exchange requests may be made in writing or by
                        telephone. The Funds reserve the right to revise or
                        terminate the exchange privilege and its provisions,
                        limit the amount of or reject any exchange, as deemed
                        necessary, at any time, without prior notice.
- --------------------------------------------------------------------------------
 
   
EXCHANGE
PRIVILEGE
LIMITATIONS             The Funds' exchange privilege is not intended to afford
                        investors a way to speculate on short-term movements in
                        the market. Accordingly, in order to prevent excessive
                        use of the exchange privilege that may potentially
                        disrupt the management of the Funds and increase
                        transaction costs, the Funds have established a policy
                        of limiting excessive exchange activity. With regard to
                        Vanguard Institutional Index Fund, exchange activity
                        will not be deemed excessive if limited to one
                        substantive exchange redemption per calendar year, taken
                        from assets that have been invested in the Fund for
                        periods of one year or longer. With regard to the other
                        Funds, exchange activity will not be deemed excessive if
                        limited to two substantive exchange redemptions (at
                        least 30 days apart) from a Fund during any twelve month
                        period. The Funds are designed for long-term investors.
    
- --------------------------------------------------------------------------------
 
IMPORTANT
INFORMATION
ABOUT TELEPHONE
TRANSACTIONS            The ability to initiate exchanges by telephone is
                        automatically established on your account unless you
                        request in writing that telephone transactions on your
                        account not be permitted. The ability to initiate wire
                        redemptions by telephone will be established on your
                        account only if you specifically elect this option in
                        writing.
 
                                       30
<PAGE>   34
 
                        To protect your account from losses resulting from
                        unauthorized or fraudulent telephone instructions,
                        Vanguard generally adheres to the following security
                        procedures:
 
                        1. SECURITY CHECK. To request a transaction by
                           telephone, the caller must identify (i) the fund
                           name; and (ii) the 10-digit account number.
 
                        2. PAYMENT POLICY. The proceeds of any telephone
                           redemption by wire will be made only in accordance
                           with the shareowner's prior written instructions.
 
                        Neither the Fund nor Vanguard will be responsible for
                        the authenticity of transaction instructions received by
                        telephone, provided that reasonable security procedures
                        have been followed. Vanguard believes that the security
                        procedures described above are reasonable, and that if
                        such procedures are followed, you will bear the risk of
                        any losses resulting from unauthorized or fraudulent
                        telephone transactions on your account.
- --------------------------------------------------------------------------------
 
OTHER ACCOUNT
INFORMATION             For corporate investors, a current corporate resolution
                        must be maintained on file at Vanguard at all times. Any
                        revisions to a corporate resolution must be submitted to
                        your Service Representative at Vanguard.
 
                        To change the registration of an account, a request must
                        be submitted in writing to Vanguard and include the
                        following information: the account number and fund name
                        and class, authorized signatures, any applicable
                        signature guarantees, and other supporting legal
                        documents as necessary.
 
                        All requests should be mailed to the following address:
 
                                          Vanguard Financial Center
                                          Attn: Institutional Investor Services
                                          P.O. Box 1472
                                          Valley Forge, PA 19482-1472
 
                                       31
<PAGE>   35
 
   
<TABLE>
<S>              <C>                                    <C>
                 ---------------------------
                 THE VANGUARD GROUP
                   INSTITUTIONAL DIVISION
                 P.O. Box 2900
                 Valley Forge, PA 19482

                 FOR PARTICIPANTS IN
                   EMPLOYER-SPONSORED PLANS
                 1-800-523-1188

                 FOR OTHER INSTITUTIONAL
                   INVESTORS
                 1-800-523-1036

                 TRANSFER AGENT:
                 The Vanguard Group, Inc.
                 Vanguard Financial Center
                 Valley Forge, PA 19482

                 ELECTRONIC ACCESS TO THE
                   VANGUARD MUTUAL FUND
                   EDUCATION AND INFORMATION
                   CENTER
                 World Wide Web
                 http://www.vanguard.com

                 E-MAIL
                 [email protected]
 
     I009
</TABLE>
    
<PAGE>   36
 
     INFORMATION CONTAINED HEREIN IS SUBJECT TO COMPLETION OR AMENDMENT. A
     REGISTRATION STATEMENT RELATING TO THESE SECURITIES HAS BEEN FILED WITH THE
     SECURITIES AND EXCHANGE COMMISSION. THE INSTITUTIONAL PLUS SHARES OF THESE
     SECURITIES MAY NOT BE SOLD NOR MAY ANY OFFERS TO BUY BE ACCEPTED PRIOR TO
     THE TIME THE REGISTRATION STATEMENT BECOMES EFFECTIVE. THIS STATEMENT OF
     ADDITIONAL INFORMATION DOES NOT CONSTITUTE A PROSPECTUS. TO OBTAIN A
     CURRENTLY EFFECTIVE PROSPECTUS FOR THE INSTITUTIONAL SHARES OF VANGUARD
     INSTITUTIONAL INDEX FUND, PLEASE CONTACT VANGUARD INSTITUTIONAL INVESTOR
     SERVICES AT 1-800-523-1036.
 
                             SUBJECT TO COMPLETION
                PRELIMINARY STATEMENT OF ADDITIONAL INFORMATION
                               DATED MAY 21, 1997
 
                                     PART B
 
                       VANGUARD INSTITUTIONAL INDEX FUND
 
                      STATEMENT OF ADDITIONAL INFORMATION
 
   
                                  JULY 7, 1997
    
 
   
This Statement is not a prospectus but should be read in conjunction with the
Fund's Prospectus dated July 7, 1997, as may be amended from time to time. To
obtain the Prospectus, please call:
    
 
                   INSTITUTIONAL INVESTOR SERVICES DEPARTMENT
                                 1-800-523-8066
 
                               TABLE OF CONTENTS
 
   
<TABLE>
<CAPTION>
                                                                                            PAGE
                                                                                            ----
<S>                                                                                         <C>
Investment Objective and Policies.........................................................   B-1
Investment Limitations....................................................................   B-4
Purchase of Shares........................................................................   B-5
Redemption of Shares......................................................................   B-6
Management and Advisory Services..........................................................   B-7
Portfolio Transactions....................................................................   B-9
Description of Shares and Voting Rights...................................................   B-9
Financial Statements......................................................................  B-10
Yield and Total Return....................................................................  B-10
Performance Measures......................................................................  B-10
</TABLE>
    
 
                       INVESTMENT OBJECTIVE AND POLICIES
 
REPURCHASE AGREEMENTS  The Fund may invest in repurchase agreements with
commercial banks, brokers or dealers to generate income from its excess cash
balances. A repurchase agreement is an agreement under which the Fund acquires a
money market instrument (generally a security issued by the U.S. Government or
an agency thereof, a banker's acceptance or a certificate of deposit) from a
commercial bank, broker or dealer, subject to resale to the seller at an agreed
upon price and date (normally, the next business day). A repurchase agreement
may be considered a loan collateralized by securities. The resale price reflects
an agreed upon interest rate effective for the period the instrument is held by
the Fund and is unrelated to the interest rate on the underlying instrument. In
these transactions, the securities acquired by the Fund (including accrued
interest earned thereon) must have a total value in excess of the value of the
repurchase agreement and are held by the Fund's custodial bank until
repurchased. In addition, the Board of Trustees will monitor the Fund's
repurchase agreement transactions generally and will establish guidelines and
standards for review of the creditworthiness of any bank, broker or dealer party
to a repurchase agreement with the Fund. No more than an aggregate of 15% of the
Fund's assets, at the time of investment, will be invested in repurchase
agreements having maturities longer than seven days and securities subject to
legal or contractual restrictions on resale for which there are no readily
available market quotations. From time to time, the Fund's Board of Directors
may determine that certain restricted securities known as Rule 144A securities
are liquid and not subject to the 15% limitation described above.
 
The use of repurchase agreements involves certain risks. For example, if the
other party to the agreement defaults on its obligation to repurchase the
underlying security at a time when the value of the security has declined, the
Fund may incur a loss upon disposition of the security. If the other party to
the agreement becomes insolvent and subject to liquidation or reorganization
under the Bankruptcy Code or other laws, a court may determine that the
underlying security is collateral for a loan by the Fund not within the control
of the Fund and therefore the Fund may not be able to substantiate its interest
in the underlying security and may be deemed an unsecured creditor of the other
party to the agreement. While the Fund's management acknowledges these risks, it
is expected that they can be controlled through careful monitoring procedures.

 
                                                                            B-1
<PAGE>   37
 
LENDING OF SECURITIES  The Fund may lend its securities on a short-term or
long-term basis to qualified institutional investors who need to borrow
securities in order to complete certain transactions, such as covering short
sales, avoiding failures to deliver securities, or completing arbitrage
operations. By lending its portfolio securities, the Fund attempts to increase
its net investment income through the receipt of interest on the loan. Any gain
or loss in the market price of the securities loaned that might occur during the
term of the loan would be for the account of the Fund. The Fund may lend its
portfolio securities to qualified brokers, dealers, banks or other financial
institutions, so long as the terms, the structure and the aggregate amount of
such loans are not inconsistent with the Investment Company Act of 1940, or the
Rules and Regulations or interpretations of the Securities and Exchange
Commission (the "Commission") thereunder, which currently require that (a) the
borrower pledge and maintain with the Fund collateral consisting of cash, a
letter of credit issued by a domestic U.S. bank, or securities issued or
guaranteed by the United States Government having at all times not less than
100% of the value of the securities loaned, (b) the borrower add to such
collateral whenever the price of the securities loaned rises (i.e., the borrower
"marks to the market" on a daily basis), (c) the loan be made subject to
termination by the Fund at any time and (d) the Fund receive reasonable interest
on the loan (which may include the Fund's investing any cash collateral in
interest-bearing short-term investments), any distribution on the loaned
securities and any increase in their market value. Loan arrangements made by the
Fund will comply with all other applicable regulatory requirements, including
the rules of the New York Stock Exchange, which rules presently require the
borrower, after notice, to redeliver the securities within the normal settlement
time of three business days. All relevant facts and circumstances, including the
creditworthiness of the broker, dealer or institution, will be considered in
making decisions with respect to the lending of securities, subject to review by
the Board of Trustees.
 
At the present time, the Staff of the Commission does not object if an
investment company pays reasonable negotiated fees in connection with loaned
securities, so long as such fees are set forth in a written contract and
approved by the investment company's trustees. In addition, voting rights pass
with the loaned securities, but if a material event will occur affecting an
investment on loan, the loan must be called and the securities voted.
 
FUTURES CONTRACTS  The Fund may enter into futures contracts, options, and
options on futures contracts for the purpose of remaining fully invested and
reducing transactions costs. Futures contracts provide for the future sale by
one party and purchase by another party of a specified amount of a specific
security at a specified future time and at a specified price. Futures contracts
that are standardized as to maturity date and underlying financial instrument
are traded on national futures exchanges. Futures exchanges and trading are
regulated under the Commodity Exchange Act by the Commodity Futures Trading
Commission ("CFTC"), a U.S. Government Agency. Assets committed to futures
contracts will be segregated at the Fund's custodian bank to the extent required
by law.
 
Although futures contracts by their terms call for actual delivery or acceptance
of the underlying securities, in most cases the contracts are closed out before
the settlement date without the making or taking of delivery. Closing out an
open futures position is done by taking an opposite position ("buying" a
contract that has previously been "sold," or "selling" a contract previously
purchased) in an identical contract to terminate the position. Brokerage
commissions are incurred when a futures contract is bought or sold.
 
Futures traders are required to make a good faith margin deposit in cash or
government securities with a broker or custodian to initiate and maintain open
positions in futures contracts. A margin deposit is intended to assure
completion of the contract (delivery or acceptance of the underlying security)
if it is not terminated prior to the specified delivery date. Minimal initial
margin requirements are established by the futures exchange and may be changed.
Brokers may establish deposit requirements that are higher than the exchange
minimums. Futures contracts are customarily purchased and sold on deposits which
may range upward from less than 5% of the value of the contract being traded.
 
After a futures contract position is opened, the value of the contract is marked
to market daily. If the futures contract price changes to the extent that the
margin on deposit does not satisfy margin requirements, payment of additional
"variation" margin will be required. Conversely, change in the contract value
may reduce the required margin, resulting in a repayment of excess margin to the
contract holder. Variation margin payments
 
                                       B-2
<PAGE>   38
 
are made to and from the futures broker for as long as the contract remains
open. The Fund expects to earn interest income on its margin deposits.
 
The Fund will only use futures contracts and options to simulate full investment
in the underlying index while retaining a cash balance for Fund management
purposes.
 
Regulations of the CFTC applicable to the Fund require that all of its futures
transactions constitute bona fide hedging transactions. The Fund will only sell
futures contracts to protect securities it owns against price declines or
purchase contracts to protect against an increase in the price of securities it
intends to purchase. As evidence of this hedging interest, the Fund expects that
approximately 75% of its futures contract purchases will be "completed"; that
is, equivalent amounts of related securities will have been purchased or are
being purchased by the Fund upon sale of open futures contracts.
 
Although techniques other than the sale and purchase of futures contracts could
be used to control the Fund's exposure to market fluctuations, the use of
futures contracts may be a more effective means of hedging this exposure. While
the Fund will incur commission expenses in both opening and closing out futures
positions, these costs are lower than transaction costs incurred in the purchase
and sale of the underlying securities.
 
RESTRICTIONS ON THE USE OF FUTURES CONTRACTS  The Fund will not enter into
futures contract transactions to the extent that, immediately thereafter, the
sum of its initial margin deposits on open contracts exceeds 5% of the market
value of its total assets.
 
RISK FACTORS IN FUTURES TRANSACTIONS  Positions in futures contracts may be
closed out only on an Exchange that provides a secondary market for such
futures. However, there can be no assurance that a liquid secondary market will
exist for any particular futures contract at any specific time. Thus, it may not
be possible to close a futures position. In the event of adverse price
movements, the Fund would continue to be required to make daily cash payments to
maintain its required margin. In such situations, if the Fund has insufficient
cash, it may have to sell portfolio securities to meet daily margin requirements
at a time when it may be disadvantageous to do so. In addition, the Fund may be
required to make delivery of the instruments underlying futures contracts it
holds. The inability to close options and futures positions could also have an
adverse impact on the ability to hedge it effectively.
 
The Fund will minimize the risk that it will be unable to close out a futures
contract by only entering into futures that are traded on national futures
exchanges and for which there appears to be a liquid secondary market.
 
The risk of loss in trading futures contracts in some strategies can be
substantial, due both to the low margin deposits required and the extremely high
degree of leverage involved in futures pricing. As a result, a relatively small
price movement in a futures contract may result in immediate and substantial
loss (as well as gain) to the investor. For example, if at the time of purchase,
10% of the value of the futures contract is deposited as margin, a subsequent
10% decrease in the value of the futures contract would result in a total loss
of the margin deposit, before any deduction for the transaction costs, if the
account were then closed out. A 15% decrease would result in a loss equal to
150% of the original margin deposit if the contract were closed out. Thus, a
purchase or sale of a futures contract may result in losses in excess of the
amount invested in the contract. However, because the futures strategy of the
Fund is engaged in only for hedging purposes, the Fund's officers do not believe
that the Fund is subject to the risks of loss frequently associated with futures
transactions. The Fund would presumably have sustained comparable losses if,
instead of the futures contract, it had invested in the underlying financial
instrument and sold it after the decline.
 
Utilization of futures transactions by the Fund does involve the risk of
imperfect or no correlation where the securities underlying futures contracts
have different maturities than the portfolio securities being hedged. It is also
possible that the Fund could both lose money on futures contracts and also
experience a decline in value of its portfolio securities. There is also the
risk of loss by the Fund of margin deposits in the event of bankruptcy of a
broker with whom the Fund has an open position in a futures contract or related
option.
 
Most futures exchanges limit the amount of fluctuation permitted in futures
contract prices during a single trading day. The daily limit establishes the
maximum amount that the price of a futures contract may vary either up or down
from the previous day's settlement price at the end of a trading session. Once
the daily limit
 
                                       B-3
<PAGE>   39
 
has been reached in a particular type of contract, no trades may be made on that
day at a price beyond that limit. The daily limit governs only price movement
during a particular trading day and therefore does not limit potential losses,
because the limit may prevent the liquidation of unfavorable positions. Futures
contract prices have occasionally moved to the daily limit for several
consecutive trading days with little or no trading, thereby preventing prompt
liquidation of future positions and subjecting some futures traders to
substantial losses.
 
FEDERAL TAX TREATMENT OF FUTURES CONTRACTS  The Fund is required for federal
income tax purposes to recognize as income for each taxable year its net
unrealized gains and losses on certain futures contracts as of the end of the
year as well as those actually realized during the year. In most cases, any gain
or loss recognized with respect to a futures contract is considered to be 60%
long-term capital gain or loss and 40% short-term capital gain or loss, without
regard to the holding period of the contract. Furthermore, sales of futures
contracts which are intended to hedge against a change in the value of
securities held by the Fund may affect the holding period of such securities
and, consequently, the nature of the gain or loss on such securities upon
disposition. The Fund may be required to defer the recognition of losses on
futures contracts to the extent of any unrecognized gains on related positions
held by the Fund.
 
In order for the Fund to continue to qualify for federal income tax treatment as
a regulated investment company, at least 90% of its gross income for a taxable
year must be derived from qualifying income; i.e., dividends, interest, income
derived from loans of securities, gains from the sale of securities or of
foreign currencies or other income derived with respect to the Fund's business
of investing in securities. In addition, gains realized on the sale or other
disposition of securities held for less than three months must be limited to
less than 30% of the Fund's annual gross income. It is anticipated that any net
gain realized from the closing out of futures contracts will be considered gain
from the sale of securities and therefore be qualifying income for purposes of
the 90% requirement. In order to avoid realizing excessive gains on securities
held less than three months, the Fund may be required to defer the closing out
of futures contracts beyond the time when it would otherwise be advantageous to
do so. It is anticipated that unrealized gains on futures contracts, which have
been open for less than three months as of the end of the Fund's fiscal year and
which are recognized for tax purposes, will not be considered gains on sales of
securities held less than three months for the purpose of the 30% test.
 
The Fund will distribute to shareholders annually any net capital gains which
have been recognized for federal income tax purposes (including unrealized gains
at the end of the Fund's fiscal year) on futures transactions. Such
distributions will be combined with distributions of capital gains realized on
the Fund's other investments and shareholders will be advised on the nature of
the distributions.
 
                             INVESTMENT LIMITATIONS
 
Except as indicated otherwise below, the following restrictions and fundamental
policies cannot be changed without approval of the holders of a majority of the
outstanding shares of the Fund (as defined in the Investment Company Act of 1940
(the "1940 Act")). The Fund may not under any circumstances:
 
 1) change its investment objective, which is to provide investment results that
    correspond to the price and yield performance of publicly-traded common
    stocks;
 
 2) change its investment policy, which is to attempt to duplicate the
    performance of Standard & Poor's 500 Composite Stock Price Index by owning
    as many of the 500 stocks contained in the index as is feasible;
 
 3) invest in commodities or purchase real estate, although it may purchase
    securities of companies which deal in real estate or interests therein
    except that the Fund may invest in stock index futures contracts, stock
    options and options on stock index futures contracts to that extent that not
    more than 5% of the Fund's assets are required as margin deposit for such
    futures contracts;
 
 4) lend money to any person except (i) by purchasing a portion of an issue of
    short-term debt securities or similar obligations (including repurchase
    agreements) which are publicly distributed or customarily purchased by
    institutional investors, and (ii) as provided under "Lending of Securities";
 
 5) purchase securities on margin or sell securities short except as described
    in limitation number "3";
 
                                       B-4
<PAGE>   40
 
 6) with respect to 75% of the Fund's assets, purchase more than 10% of the
    outstanding voting securities of any company, or purchase securities of any
    issuer (except obligations of the United States Government and its
    instrumentalities), if as a result, more than 5% of the value of the
    Portfolio's total assets would be invested in the securities of such issuer;
 
 7) borrow money, except that the Fund may borrow from banks (or through reverse
    repurchase agreements), for temporary or emergency (not leveraging)
    purposes, including the meeting of redemption requests which might otherwise
    require the untimely disposition of securities, in an amount not exceeding
    15% of the value of the Fund's net assets (including the amount borrowed and
    the value of any outstanding reverse repurchase agreements) at the time the
    borrowing is made. Whenever borrowings exceed 5% of the value of the Fund's
    net assets, the Fund will not make any additional investments;
 
 8) pledge, mortgage or hypothecate the Fund's assets to an extent greater than
    5% of its total assets;
 
 9) invest in securities of other investment companies, except as they may be
    acquired as a part of a merger, consolidation or acquisition of assets
    approved by the Fund's shareholders or otherwise to the extent permitted by
    Section 12 of the 1940 Act. The Fund will invest only in investment
    companies which have investment objectives and policies consistent with
    those of the Fund;
 
10) invest for the purpose of controlling management of any company;
 
11) engage in the business of underwriting securities issued by other persons,
    except to the extent that the Fund may technically be deemed to be an
    underwriter under the Securities Act of 1933, as amended, in disposing of
    portfolio securities;
 
12) invest more than 25% of the value of its total assets in any one industry;
 
13) invest in put, call, straddle or spread options or in interests in oil, gas
    or other mineral exploration or development programs, except as set forth in
    limitation number "3" above; and
 
14) purchase or otherwise acquire any security if, as a result, more than 15% of
    its net assets would be invested in securities that are illiquid.
 
These investment limitations are considered at the time investment securities
are purchased. Notwithstanding these limitations, the Fund may own all or any
portion of the securities of, or make loans to, or contribute to the costs or
other financial requirements of any company which will be wholly owned by the
Fund and one or more other investment companies and is primarily engaged in the
business of providing, at-cost, management, administrative, distribution or
related services to the Fund and other investment companies.
 
                               PURCHASE OF SHARES
 
The Fund reserves the right in its sole discretion (i) to suspend the offerings
of its shares, (ii) to reject purchase orders when in the judgment of management
such rejection is in the best interest of the Fund, and (iii) to reduce or waive
the minimum investment for or any other restrictions on initial and subsequent
investments for certain fiduciary accounts or under circumstances where certain
economies can be achieved in sales of the Fund's shares.
 
EXCHANGE OF SECURITIES FOR SHARES OF THE FUND  In certain circumstances, shares
of the Fund may be purchased in exchange for common stocks. Such common stocks
must be included in the appropriate Index and have a market value in excess of
$10,000. Securities accepted by the Fund will be valued as set forth in the
Fund's prospectus as of the time of the next determination of net asset value
after such acceptance. Shares of the Fund are issued at net asset value
determined as of the same time. All dividends, subscription, or other rights
which are reflected in the market price of accepted securities at the time of
valuation become the property of the Fund and must be delivered to the Fund by
the investor upon receipt from the issuer. A gain or loss for Federal income tax
purposes would be realized by the investor upon the exchange depending upon the
cost of the securities tendered.
 
The Fund will not accept securities in exchange for its shares unless: (1) such
securities are, at the time of the exchange, included in the Fund; (2) such an
exchange will not cause the Fund's weightings to become imbalanced with respect
to the weightings of the stocks included in the Index; (3) the investor
represents and
 
                                       B-5
<PAGE>   41
 
agrees that all securities offered to the Fund are not subject to any
restrictions upon their sale by the Fund under the Securities Act of 1933, or
otherwise; (4) such securities are traded in an unrelated transaction with a
quoted sales price on the same day the exchange valuation is made; (5) the
quoted sales price used as a basis of valuation is representative (i.e., one
that does not involve a trade of substantial size which artificially influences
the price of the security); and (6) the value of any such security being
exchanged will not exceed 5% of the Fund's net assets immediately prior to the
transaction.
 
Investors interested in such purchases should contact the Fund.
 
                              REDEMPTION OF SHARES
 
The Fund may suspend redemption privileges or postpone the date of payment (i)
during any period that the New York Stock Exchange is closed, or trading on the
Exchange is restricted as determined by the Securities and Exchange Commission
(the "Commission"), (ii) during any period when an emergency exists as defined
by the rules of the Commission as a result of which it is not reasonably
practicable for the Fund to dispose of securities owned by it, or fairly to
determine the value of its assets, and (iii) for such other periods as the
Commission may permit.
 
No charge is made by the Fund for redemptions. Any redemption may be more or
less than the shareholder's cost depending on the market value of the securities
held.
 
The Fund has made an election with the Commission to pay in cash all redemptions
requested by any shareholder of record limited in amount during any 90-day
period to the lesser of $250,000 or 1% of the net assets of the Fund at the
beginning of such period. Such commitment is irrevocable without the prior
approval of the Commission. Redemptions in excess of the above limits may be
paid in whole or in part investment securities or in cash as the Fund may deem
appropriate, however, payment will be made wholly in cash unless the Trustees
believe that economic or market conditions exist which would make such a
practice detrimental to the best interests of the Fund. If redemptions are paid
in investment securities, such securities will be valued as set forth in the
Prospectus under "The Share Price of the Fund and each Portfolio," and a
redeeming shareholder would normally incur brokerage expenses if he converted
these securities to cash.
 
                                       B-6
<PAGE>   42
 
                        MANAGEMENT AND ADVISORY SERVICES
 
TRUSTEES AND OFFICERS
 
The Officers of the Fund manage its day-to-day operations and are responsible to
the Fund's Trustees. The Trustees set broad policies for the Fund and choose its
Officers. The following is a list of Trustees and Officers of the Fund and a
statement of their present positions and principal occupations during the past
five years. The mailing address of the Fund's Trustees and Officers is Post
Office Box 876, Valley Forge, PA 19482.
 
JOHN C. BOGLE, Chairman and Trustee*
     Chairman and Director of The Vanguard Group, Inc., and of each of the
     investment companies in The Vanguard Group; Director of The Mead
     Corporation, General Accident Insurance, and Chris-Craft Industries, Inc.
 
JOHN J. BRENNAN, President, Chief Executive
Officer & Trustee*
     President, Chief Executive Officer and Director of The Vanguard Group,
     Inc., and of each of the other investment companies in The Vanguard Group.
 
ROBERT E. CAWTHORN, Trustee
   
     Chairman Emeritus and Director of Rhone-Poulenc Rorer, Inc.; Managing
     Director of Global Health Care Partners/DLJ Merchant Banking Partners;
     Director of Sun Company, Inc.; and Westinghouse Electric Corporation.
    
 
BARBARA BARNES HAUPTFUHRER, Trustee
   
     Director of The Great Atlantic and Pacific Tea Company, Ikon Business
     Solutions, Inc., Raytheon Company, Knight-Ridder, Inc., and Massachusetts
     Mutual Life Insurance Co. and Trustee Emerita of Wellesly College.
    
 
BRUCE K. MACLAURY, Trustee
     President Emeritus of The Brookings Institution; Director of American
     Express Bank, Ltd., The St. Paul Companies, Inc. and National Steel
     Company.
 
BURTON G. MALKIEL, Trustee
     Chemical Bank Chairman's Professor of Economics, Princeton University;
     Director of Prudential Insurance Co. of America, Amdahl Corporation, Baker
     Fentress & Co., The Jeffrey Co. and Southern New England Communications
     Company.
ALFRED M. RANKIN, JR., Trustee
     Chairman, President and Chief Executive Officer of NACCO Industries, Inc.;
     Director of The BFGoodrich Company, and The Standard Products Company.
 
JOHN C. SAWHILL, Trustee
     President and Chief Executive Officer, the Nature Conservancy; formerly,
     Director and Senior Partner, McKinsey & Co.; President, New York
     University; Director of Pacific Gas and Electric Company, Procter & Gamble
     Company and NACCO Industries.
 
JAMES O. WELCH, JR., Trustee
     Retired Chairman of Nabisco Brands, Inc., retired Vice Chairman and
     Director of RJR Nabisco; Director of TECO Energy, Inc.; and Director of
     Kmart Corporation.
 
J. LAWRENCE WILSON, Trustee
     Chairman and Chief Executive Officer of Rohm & Hass Company; Director of
     Cummins Engine Company; and Trustee of Vanderbilt University.
 
RAYMOND J. KLAPINSKY, Secretary*
     Senior Vice President and Secretary of The Vanguard Group, Inc.; Secretary
     of each of the investment companies in The Vanguard Group.
 
RICHARD F. HYLAND, Treasurer*
     Treasurer of The Vanguard Group, Inc., and of each of the investment
     companies in The Vanguard Group.
 
KAREN E. WEST, Controller*
     Principal of The Vanguard Group, Inc.; Controller of each of the investment
     companies in The Vanguard Group.
 
- ---------------
 
*Mr. Bogle and the Officers of the Fund are "interested persons" as defined in
 the Investment Company Act of 1940.
 
                                       B-7
<PAGE>   43
 
REMUNERATION OF TRUSTEES AND OFFICERS
 
The Fund's Trustees and Officers receive no direct remuneration from the Fund.
However, the Trustees do receive remuneration for their service as Directors or
Trustees of the Funds comprising the Vanguard Group of Investment Companies. The
following table provides detailed information with respect to the aggregate
amounts paid or accrued for the Trustees by the Vanguard Funds for the fiscal
year ended December 31, 1996.
 
                               COMPENSATION TABLE
 
<TABLE>
<CAPTION>
                                                                ESTIMATED          TOTAL COMPENSATION
                                                             ANNUAL BENEFITS     FROM ALL VANGUARD FUNDS
                     NAMES OF TRUSTEES                       UPON RETIREMENT       PAID TO TRUSTEES(2)
- -----------------------------------------------------------  ---------------     -----------------------
<S>                                                          <C>                 <C>
John C. Bogle(1)                                                      --                      --
John J. Brennan(1)                                                    --                      --
Barbara Barnes Hauptfuhrer                                       $15,000                 $65,000
Robert E. Cawthorn                                               $13,000                 $65,000
Bruce K. MacLaury                                                $12,000                 $60,000
Burton G. Malkiel                                                $15,000                 $65,000
Alfred M. Rankin, Jr.                                            $15,000                 $65,000
John C. Sawhill                                                  $15,000                 $65,000
James O. Welch, Jr.                                              $15,000                 $65,000
J. Lawrence Wilson                                               $15,000                 $65,000
</TABLE>
 
- ---------------
 
(1) As "Interested Trustees," Messrs. Bogle and Brennan receive no compensation
    for their service as Trustees.
(2) The amounts reported in this column reflect the total compensation paid to
    each Trustee for their service as Director or Trustee of 34 Vanguard funds
    (33 in the case of Mr. Malkiel; 27 in the case of Mr. MacLaury).
 
THE VANGUARD GROUP
 
The Fund currently employs The Vanguard Group, Inc. ("Vanguard") to provide
management, administrative and investment advisory services. Vanguard also
provides virtually all of the corporate management, administrative, and
distribution services for The Vanguard Group of Investment Companies, a family
of more than 30 investment companies with more than 90 distinct investment
portfolios and total assets in excess of $250 billion. Vanguard also provides
investment advisory services to certain Vanguard Funds.
 
Vanguard employs a supporting staff of management and administrative personnel
needed to provide the requisite services to the Fund and also furnishes the Fund
with the necessary office space, furnishings and equipment.
 
The Vanguard Group adheres to a Code of Ethics established pursuant to Rule
17j-1 under the Investment Company Act of 1940. The Code is designed to prevent
unlawful practices in connection with the purchase or sale of securities by
persons associated with Vanguard. Under Vanguard's Code of Ethics certain
officers and employees of Vanguard who are considered access persons are
permitted to engage in personal securities transactions. However, such
transactions are subject to procedures and guidelines substantially similar to
those recommended by the mutual fund industry and approved by the U.S.
Securities and Exchange Commission.
 
The Fund receives all investment advisory services from Vanguard's Core
Management Group. The Core Management Group manages the investment and
reinvestment of the Fund's assets and continuously reviews, supervises, and
administers the Fund's investment program with respect to those assets. The Core
Management Group discharges its responsibilities subject to the control of the
Officers and Trustees of the Fund.
 
The Core Management Group also provides investment advisory services to several
Vanguard Funds, including Vanguard Index Trust, Vanguard Balanced Index Fund,
Vanguard International Equity Index Fund, Vanguard REIT Index, Vanguard Treasury
Fund, the Growth and Income and Capital Appreciation Portfolios and the equity
portion of the Balanced Portfolio of Vanguard Tax-Managed Fund, the Aggressive
Growth Portfolio of Vanguard Horizon Fund, Vanguard Variable Insurance
Fund-Equity Index Portfolio, and a portion of Vanguard/Windsor II, a portion of
Vanguard/Morgan Growth Fund as well as to several indexed separate accounts.
Total assets under management by the Core Management Group were approximately
$57 billion as of December 31, 1996. The Fund is not actively managed, but is
instead administered by the Core Management Group, using computerized,
quantitative techniques.
 
                                       B-8
<PAGE>   44
 
Under the terms of the Service and Advisory Agreement, Vanguard pays all of the
Fund's expenses, except for taxes and brokerage commissions. For the years ended
December 31, 1994, 1995 and 1996, the Fund paid approximately $2,044,000,
$3,057,000, and $5,580,669, respectively, to Vanguard for services rendered
under the Service and Advisory Agreement in effect for the Fund's 1994, 1995 and
1996 fiscal year end.
 
                             PORTFOLIO TRANSACTIONS
 
In placing portfolio transactions, the Core Management Group uses its best
judgment to choose the broker most capable of providing the brokerage services
necessary to obtain best available price and most favorable execution. The full
range and quality of brokerage services available are considered in making these
determinations. In those instances where it is reasonably determined that more
than one broker can offer the brokerage services needed to obtain the best
available price and most favorable execution, consideration will be given to
those brokers which supply statistical information and provide other services in
addition to execution services to the Fund.
 
Since the Fund does not market its shares through intermediary brokers or
dealers, it is not the Fund's practice to allocate brokerage or principal
business on the basis of sales of its shares which may be made through such
firms. However, the Fund may place portfolio orders with qualified
broker-dealers who recommend the Fund to clients, and may, when a number of
brokers and dealers can provide best price and execution on a particular
transaction, consider the sale of Fund shares by a broker or dealer in selecting
among broker dealers. For the years ended December 31, 1994, 1995, and 1996, the
Fund paid $314,246, $496,995, and $862,036 respectively, in brokerage
commissions.
 
                    DESCRIPTION OF SHARES AND VOTING RIGHTS
 
The Declaration of Trust permits the Trustees to issue an unlimited number of
shares of beneficial interest, without par value. The Board of Trustees has the
power to designate one or more classes or series of shares. Currently, the Fund
is offering shares of one series, which issues two classes of shares: Vanguard
Institutional Index Fund Institutional Shares and Vanguard Institutional Index
Fund Institutional Plus Shares.
 
The shares of the Fund are fully paid and nonassessable, except as set forth
under "Shareholder and Trustee Liability," and have no preference as to
conversion, exchange, dividends, retirement or other features. The shares of the
Fund have no pre-emptive rights. The shares of the Fund have non-cumulative
voting rights, which means that the holders of more than 50% of the shares
voting for the election of Trustees can elect 100% of the Trustees if they
choose to do so. A shareholder is entitled to one vote for each full share held
(and a fractional vote for each fractional share held), then standing in his
name on the books of the Fund. On any matter submitted to a vote of
shareholders, all shares of the Fund then issued and outstanding and entitled to
vote, irrespective of the class or series, shall be voted in the aggregate and
not by class or series, except (i) when required by the Investment Company Act
of 1940, shares shall be voted by individual class or series; and (ii) when the
matter does not affect any interest of a particular class or series, then only
shareholders of the affected classes or series shall be entitled to vote
thereon.
 
The Fund will continue without limitation of time, provided however that:
 
          (1) Subject to the majority vote of the holders of shares of the Fund
     outstanding, the Trustees may sell or convert the assets of the Fund to
     another investment company in exchange for shares of such investment
     company and distribute such shares ratably among the shareholders of the
     Fund;
 
          (2) Subject to the majority vote of shares of the Fund outstanding,
     the Trustees may sell and convert into money the assets of the Fund and
     distribute such assets ratably among the shareholders of the Fund; and
 
          (3) Without the approval of the shareholders of the Fund, unless
     otherwise required by law, the Trustees may combine the assets of any two
     or more Portfolios into a single Portfolio so long as such combination will
     not have a material adverse effect upon the shareholders of such Portfolio.
 
Upon completion of the distribution of the remaining proceeds or the remaining
assets of any Portfolio as provided in paragraphs 1), 2), 3) above the Trust
shall terminate as to that Portfolio and the Trustees shall be
 
                                       B-9
<PAGE>   45
 
discharged of any and all further liabilities and duties hereunder and the
right, title and interest of all parties shall be cancelled and discharged.
 
SHAREHOLDER AND TRUSTEE LIABILITY  Under Pennsylvania law, shareholders of such
a Trust may, under certain circumstances, be held personally liable as partners
for the obligations of the Trust. Therefore, the Declaration of Trust contains
an express disclaimer of shareholder liability for acts or obligations of the
Trust and requires that notice of such disclaimer be given in each agreement,
obligation, or instrument entered into or executed by the Trust or the Trustees.
The Declaration of Trust provides for indemnification out of the Trust property
of any shareholder held personally liable (and not because of such shareholder's
acts or omissions) for the obligations of the Trust. The Declaration of Trust
also provides that the Trust shall, upon request, assume the defense of any
claim against any shareholder for any act or obligation of the Trust and satisfy
any judgment thereon. Thus, the risk of a shareholder incurring financial loss
on account of shareholder liability is limited to circumstances in which the
Trust itself would be unable to meet its obligations.
 
The Declaration of Trust further provides that the Trustees will not be liable
for errors of judgment or mistakes of fact or law, but nothing in the
Declaration of Trust protects a Trustee against any liability to which he would
otherwise be subject by reason of willful misfeasance, bad faith, gross
negligence, or reckless disregard of the duties involved in the conduct of his
office.
 
                              FINANCIAL STATEMENTS
 
The Fund's financial statements for the year ended December 31, 1996, including
the financial highlights for each of the periods, appearing in the Vanguard
Institutional Index Fund 1996 Annual Report to Shareholders, and the report
thereon of Price Waterhouse LLP, independent accountants, also appearing
therein, are incorporated by reference in this Statement of Additional
Information. The Fund's 1996 Annual Report to Shareholders is enclosed with this
Statement of Additional Information and does not include information for the
Institutional Plus Shares of the Fund because such class was not offered during
the Fund's 1996 fiscal year.
 
                             YIELD AND TOTAL RETURN
 
The average annual total return of the Institutional Shares of the Fund* for one
year, three and five years ending December 31, 1996, and since inception on July
31, 1990 was +23.06%, +19.71%, +15.21% and +15.30%, respectively. Total return
is computed by finding the average compounded rates of return over the one-,
five- and ten-year periods (or life of Fund, as applicable) set forth above that
would equate an initial amount invested at the beginning of the periods to the
ending redeemable value of the investment. The annualized yield for the
Institutional Shares of the Fund for the thirty days ended December 31, 1996 was
+1.97%.
- ---------------
 
* The Fund reserves the right to deduct a portfolio transaction fee, ranging
  from 0.08% to 0.20%, from purchases of shares of the Fund if such purchase or
  cumulative purchases are of a size that is reasonably deemed to be disruptive
  to efficient portfolio management; total return figures are not adjusted to
  reflect this transaction fee.
 
                              PERFORMANCE MEASURES
 
Vanguard may use reprinted material discussing The Vanguard Group, Inc. or any
of the member funds of The Vanguard Group of Investment Companies.
 
The Fund may from time to time use one or more of the following unmanaged
indices for comparative performance purposes.
 
STANDARD & POOR'S 500 COMPOSITE STOCK PRICE INDEX -- is a well diversified list
of 500 companies representing the U.S. Stock Market.
 
WILSHIRE 5000 EQUITY INDEX -- consists of more than 7,000 common equity
securities, covering all stocks in the U.S. for which daily pricing is
available.
 
WILSHIRE 4500 EQUITY INDEX -- consists of all stocks in the Wilshire 5000 except
for the 500 stocks in the Standard & Poor's 500 Index.
 
                                      B-10
<PAGE>   46
 
MORGAN STANLEY CAPITAL INTERNATIONAL EAFE INDEX -- is an arithmetic, market
value-weighted average of the performance of over 900 securities listed on the
stock exchanges of countries in Europe, Australia and the Far East.
 
GOLDMAN SACHS 100 CONVERTIBLE BOND INDEX -- currently includes 71 bonds and 29
preferreds. The original list of names was generated by screening for
convertible issues of $100 million or greater in market capitalization. The
index is priced monthly.
 
SALOMON BROTHERS GNMA INDEX -- includes pools of mortgages originated by private
lenders and guaranteed by the mortgage pools of the Government National Mortgage
Association.
 
SALOMON BROTHERS HIGH-GRADE CORPORATE BOND INDEX -- consists of publicly issued,
non-convertible corporate bonds rated Aa or Aaa. It is a value-weighted, total
return index, including approximately 800 issues with maturities of 12 years or
greater.
 
LEHMAN LONG-TERM TREASURY BOND -- is composed of all bonds covered by the
Shearson Lehman Hutton Treasury Bond Index with maturities of 10 years or
greater.
 
MERRILL LYNCH CORPORATE & GOVERNMENT BOND -- consists of over 4,500 U.S.
Treasury, Agency and investment grade corporate bonds.
 
LEHMAN CORPORATE (Baa) BOND INDEX -- all publicly offered fixed-rate,
nonconvertible domestic corporate bonds rated Baa by Moody's, with a maturity
longer than 1 year and with more than $25 million outstanding. This index
includes over 1,000 issues.
 
BOND BUYER MUNICIPAL BOND INDEX (20 YEAR) -- is a yield index on current coupon
high-grade general obligation municipal bonds.
 
STANDARD & POOR'S PREFERRED INDEX -- is a yield index based upon the average
yield of four high-grade, non-callable preferred stock issues.
 
NASDAQ INDUSTRIAL INDEX -- is composed of more than 3,000 industrial issues. It
is a value-weighted index calculated on price change only and does not include
income.
 
COMPOSITE INDEX -- 70% Standard & Poor's 500 Index, 30% NASDAQ Industrial Index.
 
COMPOSITE INDEX -- 65% Standard & Poor's 500 Index and 35% Lehman Long-Term
Corporate AA or Better Bond Index.
 
COMPOSITE INDEX -- 65% Lehman Long-Term Corporate AA or Better Bond Index and a
35% weighting in a blended equity composite (75% Standard & Poor's/BARRA Value
Index and 12.5% Standard & Poor's Utilities Index and 12.5% Standard and Poor's
Telephone Index).
 
LEHMAN LONG-TERM CORPORATE AA OR BETTER BOND INDEX -- consists of all publicly
issued, fixed rate, nonconvertible investment grade, dollar-denominated,
SEC-registered corporate debt rated AA or AAA.
 
LEHMAN BROTHERS AGGREGATE BOND INDEX -- is a market weighted index that contains
individually priced U.S. Treasury, agency, corporate, and mortgage pass-through
securities corporate rated BBB- or better. The Index has a market value of over
$4 trillion.
 
LEHMAN BROTHERS MUTUAL FUND SHORT (1-5) GOVERNMENT/CORPORATE INDEX -- is a
market weighted index that contains individually priced U.S. Treasury, agency,
and corporate investment grade bonds rated BBB- or better with maturities
between 1 and 5 years. The index has a market value of over $1.6 trillion.
 
LEHMAN BROTHERS MUTUAL FUND INTERMEDIATE (5-10) GOVERNMENT/CORPORATE INDEX -- is
a market weighted index that contains individually priced U.S. Treasury, agency,
and corporate securities rated BBB- or better with maturities between 5 and 10
years. The index has a market value of over $700 billion.
 
LEHMAN BROTHERS MUTUAL FUND LONG (10+) GOVERNMENT/CORPORATE INDEX -- is a market
weighted index that contains individually priced U.S. Treasury, agency, and
corporate securities rated BBB- or better with maturities greater than 10 years.
The index has a market value of over $900 billion.
 
                                      B-11
<PAGE>   47
 
                                     PART C
                       VANGUARD INSTITUTIONAL INDEX FUND
                               OTHER INFORMATION
 
ITEM 24. FINANCIAL STATEMENTS AND EXHIBITS.
(A) FINANCIAL STATEMENTS.
 
The Registrant's financial statements for the year ended December 31, 1996,
including the financial highlights for each of the respective periods presented,
appearing in the Vanguard Institutional Index Fund's 1996 Annual Report to
Shareholders and the reports thereon of Price Waterhouse LLP, independent
accountants, are incorporated by reference in this Statement of Additional
Information, from the Registrant's 1996 Annual Report to Shareholders which has
been filed with the Commission. The Financial Statements included in the Annual
Report are:
 
 1. Statement of Net Assets as of December 31, 1996.
 2. Statement of Operations for the year ended December 31, 1996.
 3. Statement of Changes in Net Assets for the years ended December 31, 1995 and
1996.
 4. Financial Highlights for the respective periods ended December 31, 1996.
 5. Notes to Financial Statements.
 6. Report of Independent Accountants.
 
(B) EXHIBITS
 
 1. Declaration of Trust.**
 2. By-Laws of Registrant.**
 3. Not Applicable.
 4. Not Applicable.
 5. Not Applicable.
 6. Not Applicable.
 7. Reference is made to the section entitled "Management and Advisory Services"
    in the Registrant's Statement of Additional Information.
 8. Form of Custody Agreement.**
 9. Form of Vanguard Service Agreement.**
10. Opinion of Counsel.**
11. Consent of Independent Accountants.*
13. Not Applicable.
14. Not Applicable.
15. Not Applicable.
16. Schedule for Computation of Performance Quotations.*
18. Registrant's Form of Multiple Class Plan Pursuant to Rule 18f-3 under the
    Investment Company Act of 1940.**
19. Powers-of-Attorney.**
27. Financial Data Schedule.*
- ---------------
 * Filed herewith.
** Previously filed.
 
ITEM 25. PERSONS CONTROLLED BY OR UNDER COMMON CONTROL WITH REGISTRANT
 
Registrant is not controlled by or under common control with any person. The
Officers of the Registrant, the investment companies in The Vanguard Group of
Investment Companies and The Vanguard Group, Inc. are identical. Reference is
made to the caption "Management and Investment Advisory Services" in the
Prospectus constituting Part A and "Management and Advisory Services" in the
Statement of Additional Information constituting Part B of this Registration
Statement.
<PAGE>   48
 
ITEM 26. NUMBER OF HOLDERS OF SECURITIES
 
As of March 31, 1996 there were 15,729 shareholders of the Fund.
 
ITEM 27. INDEMNIFICATION
 
Reference is made to Article XI of Registrant's Declaration of Trust.
 
Insofar as indemnification for liability arising under the Securities Act of
1933 may be permitted to trustees, officers and controlling persons of the
Registrant pursuant to the foregoing provisions, or otherwise, the Registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the Registrant of expenses incurred
or paid by a director, officer or controlling person of the Registrant in the
successful defense of any action, suit or proceeding) is asserted by such
trustee, officer or controlling person in connection with the securities being
registered, the Registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.
 
ITEM 28. BUSINESS AND OTHER CONNECTIONS OF INVESTMENT ADVISER
 
Investment advisory services are provided to the Registrant by The Vanguard
Group, Inc. a jointly-owned subsidiary of the Funds in the Vanguard Group of
Investment Companies. See the information concerning The Vanguard Group, Inc.
set forth in Parts A and B. For information as to any other business, vocation
or employment of a substantial nature in which each director or officer of the
Registrant's investment advisor is or has been engaged for his own account or in
the capacity of trustee, officer, employee, partner or trustee, reference is
made to Form ADV (File #801-11953) filed by it under the Investment Advisers Act
of 1940.
 
ITEM 29. PRINCIPAL UNDERWRITERS
(a) None.
(b) Not Applicable.
 
ITEM 30. LOCATION OF ACCOUNTS AND RECORDS
 
The books, accounts and other documents required by Section 31(a) under the
Investment Company Act and the rules promulgated thereunder will be maintained
in the physical possession of Registrant; Registrant's Transfer Agent, The
Vanguard Group, Inc. c/o The Vanguard Financial Center, Valley Forge, PA 19482;
and the Registrant's Custodian, CoreStates Bank, N.A., Broad and Market Streets,
Philadelphia, PA 19103.
 
ITEM 31. MANAGEMENT SERVICES
 
Other than the Service and Advisory Agreement with The Vanguard Group, Inc.
which was previously filed as an Exhibit and described in Part B hereof under
"Management and Advisory Services," the Registrant is not a party to any
management-related service contract.
 
ITEM 32. UNDERTAKINGS
 
Registrant hereby undertakes to comply with the provisions of Section 16(c) of
the Investment Company Act of 1940 in regard to shareholders' rights to call a
meeting of shareholders for the purpose of voting on the removal of directors
and to assist in shareholder communications in such matters, to the extent
required by law.
 
Registrant hereby undertakes to provide an Annual Report to Shareholders or
prospective investors, free of charge, upon request.
<PAGE>   49
 
                                   SIGNATURES
 
   
Pursuant to the requirements of the Securities Act of 1933 and the Investment
Company Act of 1940, the Registrant hereby certifies that it meets the
requirements for effectiveness pursuant to paragraph (b) of Rule 485 and has
duly caused this Post-Effective Amendment to this Registration Statement to be
signed on its behalf by the undersigned, thereunto duly authorized, in the Town
of Valley Forge and the Commonwealth of Pennsylvania, on the 24th day of June,
1997.
    
 
VANGUARD INSTITUTIONAL INDEX FUND
 
BY: (Raymond J. Klapinsky)*
    John J. Brennan, President and Chief Executive Officer
 
Pursuant to the requirements of the Securities Act of 1933, this Post-Effective
Amendment to this Registration Statement has been signed below by the following
persons in the capacities and on the date indicated:
 
BY: (Raymond J. Klapinsky)*
    John C. Bogle, Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    John J. Brennan, President, Trustee and Chief Executive Officer
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    Robert E. Cawthorn, Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    Barbara B. Hauptfuhrer, Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    Bruce K. MacLaury, Jr., Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    Burton G. Malkiel, Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    Alfred M. Rankin, Jr., Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    John C. Sawhill, Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    James O. Welch, Jr., Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    J. Lawrence Wilson, Trustee
   
    June 24, 1997
    
 
BY: (Raymond J. Klapinsky)*
    Richard F. Hyland, Treasurer and Principal
    Financial Officer and Accounting Officer
   
    June 24, 1997
    
 
* As Attorney-in-Fact, pursuant to Power-of-Attorney. See Form SE filed for the
  Windsor Funds, Inc. (File Number 2-14336), as filed with the Commission on or
  about January 23, 1990, incorporated by reference.
<PAGE>   50
 
                               INDEX TO EXHIBITS
 
<TABLE>
<S>                                                                                    <C>
Consent of Independent Accountants...................................................  EX-99.B11
Schedule for Computation of Performance Quotations...................................  EX-99.B16
Financial Data Schedule..............................................................  EX-27
</TABLE>

<PAGE>   1
 
                                                                       EX-99.B11
 
                       CONSENT OF INDEPENDENT ACCOUNTANTS
 
We hereby consent to the incorporation by reference in the Prospectus and
Statement of Additional Information constituting parts of this Post-Effective
Amendment No. 9 to the registration statement on Form N-1A (the "Registration
Statement") of our report dated January 31, 1997, relating to the financial
statements and financial highlights appearing in the December 31, 1996 Annual
Report to Shareholders of Vanguard Institutional Index Fund, which are
incorporated by reference into the Registration Statement. We also consent to
the references to us under the headings "Financial Highlights" and "General
Information" in the Prospectus and "Financial Statements" in the Statement of
Additional Information.
 
PRICE WATERHOUSE LLP
Philadelphia, PA
   
June 23, 1997
    

<PAGE>   1
 
                                                                       EX-99.B16
 
              SCHEDULE FOR COMPUTATION OF PERFORMANCE QUOTATIONS*
                       VANGUARD INSTITUTIONAL INDEX FUND
 
1. Average Annual Total Return
        P (1 + T)(n) = ERV
 
<TABLE>
   <S>          <C>     <C>
   Where:       P = a hypothetical initial payment of $1,000
                T = average annual total return
                N = number of years
                ERV = ending redeemable value at the end of the period
   EXAMPLE:
   ------------
   One Year
   --------
             P =   $1,000
             T =   +23.06%
             N =   1
           ERV =   $1,230.59
   Three Year
   ----------
             P =   $1,000
             T =   +19.71%
             N =   3
           ERV =   $1,715.47
   Five Year
   ---------
             P =   $1,000
             T =   +15.21%
             N =   5
           ERV =   $2,029.68
   Since inception**
   -----------------
             P =   $1,000
             T =   +15.30%
             N =   Since inception
           ERV =   $2,493.66
</TABLE>
 
   **July 31, 1990
 
2. YIELD (30 Days Ended December 31, 1996)
 
                  Yield = 2[(a - b +1)(6)-1]
                             -----
                             c x d
 
<TABLE>
   <C>         <S>
   Where:      a = dividends and interest paid during the period
               b = expense dollars during the period (net of reimbursements)
               c = the average daily number of shares outstanding during the period
               d = the maximum offering price per share on the last day of the period

    Example:   a = $18,928,692.58
               b = $568,457.40
               c = 160,231,749.439
               d = $70.05
           Yield = 1.97%
</TABLE>
 
   *Figures presented are as of the year ended December 31, 1996.

<TABLE> <S> <C>

<ARTICLE> 6
<CIK> 0000862084
<NAME> VANGUARD INSTITUTIONAL INDEX FUND
<MULTIPLIER> 1,000
<CURRENCY> US
       
<S>                             <C>
<PERIOD-TYPE>                   YEAR
<FISCAL-YEAR-END>                          DEC-31-1996
<PERIOD-START>                             JAN-01-1996
<PERIOD-END>                               DEC-31-1996
<EXCHANGE-RATE>                                      1
<INVESTMENTS-AT-COST>                        8,387,920
<INVESTMENTS-AT-VALUE>                      11,461,352
<RECEIVABLES>                                  119,249
<ASSETS-OTHER>                                       0
<OTHER-ITEMS-ASSETS>                                 0
<TOTAL-ASSETS>                              11,580,601
<PAYABLE-FOR-SECURITIES>                        72,917
<SENIOR-LONG-TERM-DEBT>                              0
<OTHER-ITEMS-LIABILITIES>                       81,269
<TOTAL-LIABILITIES>                            154,186
<SENIOR-EQUITY>                                      0
<PAID-IN-CAPITAL-COMMON>                     8,317,094
<SHARES-COMMON-STOCK>                          165,942
<SHARES-COMMON-PRIOR>                          115,220
<ACCUMULATED-NII-CURRENT>                       10,130
<OVERDISTRIBUTION-NII>                               0
<ACCUMULATED-NET-GAINS>                         26,442
<OVERDISTRIBUTION-GAINS>                             0
<ACCUM-APPREC-OR-DEPREC>                     3,072,749
<NET-ASSETS>                                11,426,415
<DIVIDEND-INCOME>                              199,857
<INTEREST-INCOME>                                1,047
<OTHER-INCOME>                                       0
<EXPENSES-NET>                                   5,581
<NET-INVESTMENT-INCOME>                        195,323
<REALIZED-GAINS-CURRENT>                       169,598
<APPREC-INCREASE-CURRENT>                    1,528,203
<NET-CHANGE-FROM-OPS>                        1,893,124
<EQUALIZATION>                                  14,428
<DISTRIBUTIONS-OF-INCOME>                      204,455
<DISTRIBUTIONS-OF-GAINS>                       154,705
<DISTRIBUTIONS-OTHER>                                0
<NUMBER-OF-SHARES-SOLD>                         71,930
<NUMBER-OF-SHARES-REDEEMED>                     26,039
<SHARES-REINVESTED>                              4,831
<NET-CHANGE-IN-ASSETS>                       4,752,025
<ACCUMULATED-NII-PRIOR>                          4,834
<ACCUMULATED-GAINS-PRIOR>                       11,549
<OVERDISTRIB-NII-PRIOR>                              0
<OVERDIST-NET-GAINS-PRIOR>                           0
<GROSS-ADVISORY-FEES>                                0
<INTEREST-EXPENSE>                                   0
<GROSS-EXPENSE>                                  5,581
<AVERAGE-NET-ASSETS>                         8,943,568
<PER-SHARE-NAV-BEGIN>                            57.93
<PER-SHARE-NII>                                   1.38
<PER-SHARE-GAIN-APPREC>                          11.90
<PER-SHARE-DIVIDEND>                              1.36
<PER-SHARE-DISTRIBUTIONS>                         0.99
<RETURNS-OF-CAPITAL>                                 0
<PER-SHARE-NAV-END>                              68.86
<EXPENSE-RATIO>                                   0.06
<AVG-DEBT-OUTSTANDING>                               0
<AVG-DEBT-PER-SHARE>                                 0
        

</TABLE>


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