From the Vanguard web site:
Exchanges
All open Vanguard® funds accept exchange requests online, by telephone, or by mail. However, because excessive transactions can disrupt the management of a fund and increase the fund's costs for all shareholders, Vanguard places certain limits on exchanges and other transactions.
Frequent-trading policy
If you sell or exchange shares of a Vanguard fund, you will not be permitted to buy or exchange back into the same fund, in the same account, within 60 calendar days. However, this rule does not apply to:
Vanguard money market and short-term bond funds.
Vanguard VIPER® Shares.
Transaction requests submitted by mail to Vanguard by shareholders who hold their accounts directly with us. Please note that transaction requests submitted by fax or wire are subject to the policy.
Purchases of shares with fund dividends or capital gains distributions. For example, if you reinvest dividends or capital gains, these purchases will not be blocked even if shares in the fund were sold or exchanged within the previous 60 days.
Transactions made through Vanguard’s Automatic Investment Plan, Automatic Exchange Service, Direct Deposit Service, Automatic Withdrawal Plan, Required Minimum Distribution Service, and Vanguard Small Business Online®.
Sales of shares by Vanguard to pay fund or account fees.
Transfers and re-registrations of shares within the same fund.
Purchases of shares by asset transfer or direct rollover.
Conversions of shares from one share class to another in the same fund.
Sales of shares through checkwriting.
Section 529 college savings plans, certain approved institutional portfolios and asset allocation programs, as well as Vanguard funds that invest in other Vanguard funds.
For information about the Vanguard Variable Insurance Funds’ frequent-trading policy, see the funds’ prospectus. A separate policy applies to participants in employer-sponsored retirement plans administered by Vanguard.
Note that Vanguard reserves the right to revise or terminate the exchange privilege, limit the amount of any exchange, or reject an exchange, at any time, for any reason.
Additional information regarding exchanges
If you have a Tele-Account® PIN, you may also exchange shares via our automated Tele-Account service (800-662-6273).
You may waive the telephone privilege for any Vanguard account by sending us a letter of instruction signed by all registered shareholders indicating the Vanguard accounts affected. This waiver includes telephone, Tele-Account, and online exchanges.
Exchanges, like redemptions, may be taxable events.