I met this nice old lady in one of the rice shops our tour guide brought us to. She was selling her cakes for a few Dong. Dong is the local currency. It has a semi-fixed exchange rate with the US dollar. In some hotels and restaurants you can pay in US currency, but you will need to exchange Dong to pay in small shops and bars.
Although you may get better deals in the black market, I cannot recommend this option. You may be given fake notes! Go to a bank, or if you do not find any, exchange in your hotel.