Vietnam's sole currency is Vietnam Dong - VND, which comes in notes and coins though the latter is barely seen today. Vietnam Dong notes come in denominations of:
VND 500; 1,000; 2,000;
VND 5,000; 10;000; 20,000;
VND 50,000; 100,000; 200,000; 500,000
At present the exchange rate is about VND 22,700 for 1 USD.
US dollars can also be used for your purchase while you are in larger cities' tourist areas but try to avoid doing so. Since late 2011, Vietnamese government has banned all price listing in dollars. Adding to that, Vietnamese sellers will charge you unfavourable rate if you pay in dollars.
Vietnamese currency is not common beyond Vietnam and Asia, so your banks will not have them in stocks if you are in North America or Europe. You can always get some as you land at one of the Vietnam's airport.