Vietnam's official currency is the Dong. There is a Dong-Dollar fixed exchange rate. The exchange rates for the rest of the currencies (Euros, Australian dollars, Canadian Dollars, British Pounds, etc) vary on a daily basis.
All important foreign currencies can be exchanged easily. Banks and official agencies are the place to go. In some cases you may get better deals in silver shops. Avoid exchanging in the black market, specially in the street. They may give you fake notes.
Everything costs thousands of dong. Sometimes sellers say only a number. Let's say, six. They mean six thousand dong. If a taxi driver (or any seller) says six, make sure he means six thousand dong, and not six US dollars.