Just to warn other travelers of what happened to me when trying to withdrawn money from an ATM on Pham Ngu Lao, center of backpacker district. After inserting pin code and selecting the amount, the card came out, no money or error/unauthorized message. My bank just warned me of a suspicious transaction with that amount, and I already cancelled the card.
Basically the atm is acting as a payment terminal, you select the amount and instead of withdrawing money, you're paying. I don't know if this is common in HCMC. The atm was standing inside a shop next to a Citybank atm, but unfortunately I don't remember the name of it. When I return to HCMC I will check it to warn other people of this scam.