The Dong is the official currency of Vietnam. It is divided into 10 hao, which are subsequently divided into 10 xu. The hao and xu have become so insignificant recently that they are no longer issued. Bill denominations exist in 200; 500; 1000; 2,000; 5,000; 10,000; 20,000; 50,000, 100,000, 200,000 and a newly issued 500,000 worth approx. 30 American dollars. Dong, which literally means "copper" in Vietnamese, was originally named for the currencies used when Vietnam was still part of French Indochina. After the fall of Saigon, in 1975, Vietnam changed the name of its currency to the "liberation dong". Each new dong was worth 500 of the southern dong. The unification of the country led to the unification of the currency as well. In the mid 1980's, the dong was revalued, with the new dong equal to 10 old dongs. As of June 2005, Vietnam was the second least valued currency after Romania.Beside VND, US dollar is widely accepted.
Travelers check is a good way to carry money around Vietnam because the exchange rate is fixed and it can be replaced in case of lost or stolen. Checks can also be easily exchangeable in banks.
Most major credit cards such as Visa, Master card and American Express are very usefully in the big cities and some tourist sites too. Those cards can be used in the most mid range to the high-end class hotels and some luxury restaurants. You can also get the cash advances from ATMs of the almost Vietnamese banks especially Vietcombank.
Banks in Vietnam has developed a large number of ATM networks but it is not recommended to trust completely in ATM because sometimes you cannot get cash from them especially during lunar New Year. Furthermore, many shops and restaurants done accept credit cards as a payment method.
You can bring as much as money to Vietnam as you can. There is no limit on amount of foreign currency by tourist but you must declare to the customs. At this time, the minimum amount of money for declaration is 7000$.