ATM is not everywhere

Although cash reigns supreme in Vietnam, ATMs are not located in every city. Of course, the larger towns are an exception, but even in 2018, there are just a few in places like Phong Nha and Tam Coc. So keep enough cash on hand while visiting larger cities to cover off-the-beaten-path/smaller villages.

Also, make sure you have some dollars that you can exchange for VND before you start your journey. It's preferable if you can buy Dong before you arrive, but because they're hard to come by, get some cash, preferably in USD. Euros or pounds can also be exchanged, albeit the US dollar is the most generally recognized currency after the Vietnamese dong. This is definitely one of the top Things to Know Before Traveling to Vietnam.