I spent five weeks in Vietnam (2 weeks volunteering in Hanoi with Global Volunteers and 3 weeks on an OAT tour). I also went to Cambodia. I never had any trouble using my credit union debit card in ATMs there and recommend using ATMs rather than exchanging dollars. I also used credit cards in places like hotels.
Cambodia actually uses American dollars as their currency and that’s what you get when you use an ATM. Do not ask for multiples of $100 because you are likely to get $100 bills which can be hard to break. Ask for an amount like $80 so you get smaller bills. If you bring dollars from home, the bills must be pristine! I had one $20 refused in a restaurant because of some small defect. Get the best bills your bank can find and maybe iron them (that’s what I did LOL!)
The Global Volunteer team stayed at the Hanoi L’Heritage Hotel, right in the heart of the old part of the city. It was a great location and the staff were very friendly and helpful. They have an elevator and AC. The breakfast was good and served up on the top floor with a view of the surrounding city. One note: ask for a room on upper floors because some of the rooms on lower levels are “caves” with no natural light. The “window” is up against a neighboring building.
The equivalent of Uber in Vietnam is “Grab” and it’s very inexpensive.
There are many local tour companies for day tours. The hotel can help you book these or browse sites like TripAdvisor.. A street food tour is lots of fun and will introduce you to wonderful food.
If you fly to Cambodia and then end up returning to Vietnam for your flight home, be sure to get a Vietnam visa allowing for multiple entries.