When you travel to a foreign country, the least economical way of getting local money is by using the foreign exchange stores at airports. Vietnam is not an exception. Usually a better way is to use your bank card on ATMs.
Unlike Philippines where all banks charge same rate of hefty fees on ATM withdrawal, the banks in Vietnam are very competitive on this part. They do business differently. Some charge 3% of the amount, some charge 1%, some charge a fixed amount, and some just don't charge a fee at all!
Before I left home, I did some research online, reading notes from other travellers. I compiled a list of banks, with the fees and maximum allowed amount of a withdrawal. The list is attached at the end.
For my own experience, I took money out from two different banks, Eximbank and Vietcombank. Both has a transaction limit of 3 million Vietnamese Dongs (VND). But Eximbank charges NO fee! Vietcombank charges a fee of 50,000 VND. Obviously, I'd recommend Eximbank.
Also the limit of 3 milllion is per transaction. I was able to withdrawal twice on the same machine, without re-inserting the card.
Usually the ATM will dispense bank notes of 500,000 VND. Although it's only about CAD $30, you would find it difficult for a small eatery to give you change after you buy a bowl of noodle soup for 40,000 VND. A tip is when you withdraw, instead of taking 3 million, you enter the amount of 2.9 million. In that case you will get some smaller notes.
Below is the list I compiled. The first number is fee, the second is withdrawal limit per transaction.
VP Bank: 0/8M
Saigon Bank: 0
Sacom: 1.5% or 30K, whichever higher. /3M
Vietin: 0 (Hoi An, Hue)
TP Bank: 0/3M
Eximbank: 0/3M* Cirrus
DONG ATM: 0/2M not Cirrus?
ANZ: 40K/5M (no more?)
Military Bank: 33k/3M not Cirrus?
Military Bank or MD: 15k/5M
SeABank: 50k, no warning