I remember from our last visit to Vietnam that ATMs charge you an amount of money to make a withdrawal, and it is different based on ATM. Just like in Thailand. 

The vietnamese cash we brought with us lasted about 4 days :)) we didn't plan to stock on cash for the whole period of time so something had to be done with the ATMs here. First I investigated a few ATMs, the ones that mention how much they will charge you for a withdrawal and the majority will charge from 1,5% to 3% of the withdrawal amount but not less than 30k-50k. One note here is that most of the ATMs in Hoi An will not let you withdraw more than 2 or 3 million dong at a time, so it is easy to calculate that 3% of 3M is 90 000 dong, which is not a trivial amount, it is how much we pay for our lunch (or dinner), and also for our 2 coffees :)) because cappuccino (or latte, Alena's drink) are quite pricy here from 35k - 55k depending on location. 

What I want to say is that eventually I found an ATM which takes no fee for withdrawal and lets you get 3M at a time, but it doesn't matter because you can take as many times as you want. And later on, in a fb post people were mentioning another atm which gives out 5M per time and takes no fee. 

Problem solved! awesome! now we can pay with cash everywhere because if you pay with card (in a very few places that allow it) they will also take 3% charge from you and it is a lot of money if you need to pay for housing like 10 000 000 or so.