There are two. A fixed tip of 1,500,000₫ per room — 1,000,000₫ for the madam and 500,000₫ for the waiter — whatever the group size; two rooms pay it twice. And 1,000,000₫ per hostess, only for the hostesses you ask for.
Tips are cash only, take no booking discount and carry no VAT, so bring Vietnamese dong.
Everything else can go on a card, with a fee of 3.2% on Visa and 4% on Mastercard or AmEx. The minimum spend is 3,000,000₫ for room, drinks and food together.