Cash

Every country has restrictions on the amount of cash that can be brought out of the border. If you plan to bring more than 10,000 dollars value, be prepared for custom declaration. Moreover, bringing cash overseas might not be the safest and most convenient way when you travel. It is important, however, to keep a couple hun-dred dollars in small denominations to cover the cost before you can find a bank or ATMs machine.  
The currency in Vietnam is Vietnamese Dong (VND). Foreign currencies can be exchanged at the banks, exchange bureaus or hotel reception desks. 
At the airports, train stations and tourists’ areas, there are plenty of currency exchange stalls. You should avoid the black market, no matter how competitive the offered rate is. You should check online currency exchange at Vietcombank.