C-Ray gives you some good advice, it's amazing how many places accept US dollars. But it is a good idea to exchange dollars for dong as dong is always accepted and dollars may not be. Also, the rates seem to be fairly consistent whether you're exchanging at the airport, a bank, a currency exchange or a hotel, unlike most everywhere else in the world where airports and hotels are usually not as good, the rates at both airports are good. American Express T/C's are accepted, but there are also ATMs where your American bank cards work just fine. The thing about Vietnam is the fact you don't need much money, if you're not a big drinker and don't eat at the high-end restaurants, $100.00 or so could easily last a week (not inc. hotel). Have fun.