Note that V&A does true reservations where a table is preassigned. They are not the ADR type of reservation where you have priority over walk-ins. If you do not show that table remains empty for the seating you had reserved. There is a charge if you are a no-show, but I don't know how much it is.
They do take your credit card, and the reservation agent will inform you of the cancellation policy when you make your reservation. They are very specific about the current policy when you call and you will be charged if you don't cancel within their guidelines.