For Vietnam it all depends on where you are going. Last year we went on a motorcycle tour and away from the tourist areas only the VND is accepted.
Changed some USD into local currency at the airport on arrival. We flew into Hanoi and there are a number of banks so we checked the rates and went with the best.
We also found when buying that our Vietnamese guide got a better deal than we could.
Don't think I used my credit card at all dollars and dong only.