How common is it to pay with a credit card in Vietnam? On a trip to Hanoi or Saigon, how often do I have to pay by cash? If you carry cash, do you pay in USD or VND?
Vietnam remains a cash society, and that cash is Vietnamese dong. US dollars may be accepted for larger purchases and places like airport shops.
Credit cards are accepted at hotels and touristy/upmarket restaurants, shops, etc, but definitely not by your average hole in the wall noodle shop. You can, however, use a credit card for Grab payments for taxis, food delivery, etc. There are a bunch of other cashless QR code payment systems in addition to GrabPay, but most are not practical for visitors.
I would argue that app based payments are becoming widespread in Vietnam, as they are across the Asia-Pacific. But since much of the Vietnamese economy is underground and many things such as tax are not often accounted for, one should assume that cash is required and assumed to be the main mode of payment unless an alternative mode of payment is available. And you will find that you cannot live on your card alone.
And VND is the main mode of currency for daily purchases in Vietnam, but ideally you should only carry large amounts of money in USD (so either withdraw small amounts from your atm each time, or carry USD and convert at a bureau de change). I think it is rare to use USD outside tourist orientared situations and certain large purchases.
In Vietnam, cash is still king but credit cards are becoming more widely accepted in the cities. In Hanoi and Saigon, most restaurants, shops and hotels will take credit, but smaller places like street food stalls are usually cash only.
It's a good idea to carry both US dollars and Vietnamese dong. Larger bills like $50s and $100s can be exchanged for dong at decent rates, while smaller transactions are often paid in dong. Just be aware of counterfeits if paying in dollars. Having cards and some of each currency covers you for most situations.
In Vietnam, cash is usually needed for a lot of things. Card (Visa/Master) is accepted as well but only in bigger establishments. You can use ATMs to get the cash there its good exchange rates and many of them don't charge fees for foreign card holders either... you can use the atm fee saver app, it gives a list of atms there with their fees, withdrawal limits etc.