Vietnam's currency is the Dong
There are many banknotes :100 Dongs; 200 Dongs; 500 Dongs; 1000 Dongs; 2000 Dongs; 5000 Dongs; 10,000 Dongs; 20,000 Dongs; 50,000 Dongs; 100,000 Dong; 200,000 Dong and 500,000 Dong.
Approximate value: 1 euro = 25,000 Dongs  (as of the creation of this document).  
Credit cards can be used for large purchases: in restaurants, shops, hotels in Hanoi, Saigon and some major cities. But you’ll have trouble paying with these cards in small towns or in the countryside.
ATMs are available in all cities and towns. This ATM networks accept Visa and Mastercard and will charge a bank fee of 3-5%.