Of course, the black market is not really recommended, since it is illegal and you are likely to be cheated, but it is possible to get even more there. Since then it is no longer legal to charge USD, but USD still is very much on its way to become the main currency within Vietnam. You can change back your surplus of VND when leaving the country by plane at the international airports. Currencies other than VND up to the amount of USD 3'000 are free from declaration. Above that, you'll have to declare it.
Travellers Cheques are easily changeable at banks and money changers all over the country.
Commissions are USD 1 for a USD 100 TC at Vietcom Bank, USD 2 at ANZ Bank and USD 2 service charge at the Tan Son Nhut Airport. You can get a cash advance on your credit card at many banks. There is a Visa office in Ho Chi Minh City . If you plan to travel for less than the Western prices, you should not rely on credit cards. Flugzeug steckengeblieben, doch es machte mir wenig aus.