VANGUARD/PRIMECAP FUND
                              PROSPECTUS SUPPLEMENT
                                 APRIL 22, 1998

Effective immediately, Vanguard/PRIMECAP Fund will no longer accept new accounts
and will limit  additional share purchases by current  investors.  These are the
limitations that apply to the Fund's current investors:

      -   $25,000  LIMIT.  Current  investors  may  purchase  up to  $25,000  in
          additional  Fund shares each calendar  year. The $25,000 limit applies
          to  the  aggregate   annual  share  purchases  in  all  Fund  accounts
          registered  under the same  taxpayer  identification  number or Social
          Security number.

      -   EXCLUSIONS.  Shares  purchased  before April 22, 1998,  will not count
          toward the $25,000 limit.  Also,  shares purchased through dividend or
          capital  gain  reinvestments  will never  count  toward the limit.  In
          addition,  participants in certain employer-sponsored retirement plans
          remain subject to their plan rules for share purchases.

      -   SHARE  TRANSFERS.  A current  investor may transfer  ownership of Fund
          shares to someone else, as long as the transfer equals at least $3,000
          ($1,000 for Uniform Gifts/Transfers to Minors Act accounts).

      -   ELECTRONIC  PURCHASES.  Fund shares may no longer be purchased through
          Vanguard's website or Tele-Account(R) service. However,  investors may
          continue  to use these  electronic  options  for  redemptions  of Fund
          shares.

The Fund may modify these policies  further at any time. You may call Vanguard's
Investor Information  Department at 1-800-662-7447 for more detailed information
on the Fund's share purchase policies.

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