Just about to leave Vietnam after 4 weeks and thought I would update on which ATMs were charging me for withdrawals and which weren't. For reference I was using a UK issued pre loaded Caxton FX (Visa) card. Not saying you will get the same results with your ATM cards (and it might be because was using a prepaid card) but worth a go... I was surprised by how many were free compared to when I was last in Vietnam. NB for all of these I still got the standard FX markup charges in the UK, below is about the levy by the Vietnamese banks.
Agribank - 0 fees (standalone ATM in Can Tho, max withdrawal D3,000,000)
ANZ D40,000 fee (used bank by Hoan Kiem lake, Hanoi, max withdrawal D5,000,000 - despite lots of forums saying you can get more out of these ATMs)
VIB D50,000 fee (used standalone ATM at Hanoi Airport, max withdrawal D2,000,000)