The currency in Vietnam is the Dong (abbreviated “VND” or just “d”). In Cambodia, it is Riels and in Laos, it is the Kips. Credit/debit card is increasingly widespread in big cities, but remains limited elsewhere. And though it is possible to pay in USD or EUR, it is easier and more saving to bargain in local currency.
Money
It is recommended to change your money into dong for payment of your purchases. Also, you do not need to change all at once upon your arrival because exchange facilities are widely available in big cities. Authorized banks may charge you 2% commission for this; much lower than in hotels or on black market. Make sure you recount the amount you get back, unless it is counted by a machine already.
Generally, it is not necessary to take a lot of cash with you. The cost of living in Vietnam, Laos and Cambodia is much lower than in the West and your budget depends on the comfort and style you wish to travel in. It is possible to bring with you less than 7,000 USD (or equivalent in EUR) without customs declaration. (in Vietnam) However, it is very risky to take a lot of cash with you.
Tipping
Although tipping is not mandatory in Vietnam, it is highly appreciated. The decision of whether to tip or not depends entirely on your satisfaction.