You can never use sale proceeds for a new security purchase until they become available in your money market settlement account. Even if you already have a brokerage cash account, you must apply separately for margin privileges. If you overdraw your money market settlement account when making a VBS trade, Vanguard will cover the shortfall by transferring assets from your other Vanguard fund accounts. You can buy an IPO at the offering price if you place an order with VBS 3 days in advance. When an order confirmation message appears on your computer screen, you know that your online trade has been executed.
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If the shortfall isn't made up in 5 days, your trading privileges may be denied for up to 90 days. In most cases you can invest the proceeds in a new security before the sale settles. The proceeds must be available in your account on or before the date that your new security purchase is scheduled to settle. However, you can't sell a security bought with pending proceeds to finance another purchase. It's your responsibility to transfer fund assets to cover any shortfall by 4 p.m. Eastern time on the business day before settlement.