There are plenty of ATM's that are open 24 hours. Vietcombank is the most common ATM service and accepts nearly all overseas cards. Other banks will accept cards depending on their alliances within Vietnam. If you use ANZ, you can use Vietcom and Vietin banks. If you use HSBC, there is an agreement with Techcom bank. The limit per withdrawal is 2 million VND. In larger cities it is possible to withdraw more, but in the centre, it remains capped at 2 million. You might want to consider the fees you are hiking up back home per withdrawal. Locally, you are charged a 20,000 VND fee.
It’s worth remembering that in terms of day-to-day expenses, $US100 goes a long way in Vietnam. Also, it is not advisable to carry large amounts of cash with you. Foreign tourists’ wallets and purses are a prime target for the packs of pickpockets; experts at all of the sleight-of-hand tricks of their nefarious trade.