Set up a "one-shot" automatic purchase or exchange. "this rule does not apply to: ... Transactions made through Vanguard’s Automatic Investment Plan, Automatic Exchange Service ... Automatic Withdrawal Plan." Accounts held at Vanguard have a number of automatic options: automatic investment, automatic withdrawal, and automatic exchange. You set them up under "account profile." These account features are normally used for automatically recurring transactions, e.g. "exchange $500 from VTSMX (Total Stock Market Index Fund) into VBMFX (Total Bond Market Index Fund) on the 4th of every month," but you can set up starting and ending dates that result in only a single transaction occurring. The system does not let you schedule an automatic transaction for the next day, so this introduces a day or two delay before the transaction completes.