There are a number of international banks operating in both Hanoi and Ho Chi Minh City with 24hour cash withdrawal facilities. Most ATMs enables you to get cash from VISA, MASTER CARD, Cirrus, Maestro, Plus and JCB network.
Under Vietnamese law, ATM may only dispense cash in Vietnamese Dong. Current amounts that can be withdrawn at any one time is usually very low, 2,000,000 Dong with an additional usage charge variable from 30.000d up.
In HCM City the best choice is still  Commonwealth Bank (Australia) with zero fee and a max limit of more than 10.000.000d, 4 ATMs around Bui Vien St, no plan to extend the net ouside HCMC. ANZ and HSBC have a 10.000.000d and  4,800,000d but with high fees.
Citibank ATM at HCM Airport permits withdrawals of 8,500,000 Dong. Dong can go up to 5.000.000 Dong but it only accepts card from the Visa circuit, no MasterCard.
There is nothing to stop you putting your card back in to get more money out, but remember you are still subject to your own bank’s daily cash withdrawal limits, about 500E for Maestro/MasterCard network in Europe, and you pay your bank fee every time. Also, ensure you tell your bank before travelling to Vietnam if you intend to use your card to withdraw cash from ATM’s. The machines here use the magnetic strip not the more secure chip technology.