20,000VND bills in Vietnam have a similar color and are often mistaken one for another. Do not toss it on the table because it is perceived as you being rude. It is also wise to carry more than one card. In Japan, a lot of places do not accept credit cards, even the American Express card. They accept mainly the JCB card, if they use it at all.
Most people carry cash.
The recipient will be charged a small fee. Operators respond in English. As a rule, do not touch Thai people on the head, ie. stroking the children's hair. This happens quite often particularly with mineral water bottles. When renting a motobike in Vietnam or Thailand, be aware that there is no insurance coverage for anything.