I'd say the best way for travellers to pay for things in Vietnam is cash right now
While credit card is accepted in many places in major cities like Hanoi and Ho Chi Minh City, you will still have a lot of trouble getting around without enough cash in hand
Mobile payment apps are also becoming more popular, but as travellers you won't be able to use them without a Vietnamese bank account
How to get cash:
Vietnamese currency is Dong (VND). 1 USD = 23,000 VND
The best way to get cash in Vietnam is to bring USD or Euros and go to a gold shop to exchange money (gold shops are literally shops that sell gold jewellery but most of them offer exchange services at the best rates in the country)
You can also exchange money at banks and money exchangers in airports. The rates you get at airports are not bad at all, comparable to banks in the city and only slightly worse than gold shops
If you run out of cash, you can always use ATMs to get more. This will be expensive as ATMs typically charge you 30,000 - 50,000 VND per withdrawal, and maximum 2-3 million VND per withdrawal
How much cash to bring:
This will heavily depend on your travel style
Aside from the cost of flight and hotel, you can travel in Vietnam for a little as $10USD (250,000 VND)per day if you're tight with your budget. I'd say most people should budget $25-$35USD (600,000VND - 850,000 VND) in cash per day in spending money, and bring a Visa or MasterCard as well to pay for any unexpected large expenses or occasional splurges
Where you can use credit cards:
As I said above, if you're in major cities like Hanoi and HCMC, or major tourist towns like Ha Long Bay and Da Nang, you will actually find many places that accept credit cards
The most commonly accepted cards are Visa, MasterCard, and JCB
The types of places that accept cards are:
Hotel chains and airlines
Tour agencies
Pre-booked local transportation and private transfers
Grab taxi
Mid to upper scale restaurants
Upscale supermarkets
Stores in shopping malls
Modern movie theaters
Other than these places, you will hardly anyone else accepting credit card for payment. If you want to get an in-depth experience of Vietnam, make use of public transportation, go to markets, and so on, you will need to pay for all of them with cash
Mobile payment:
Mobile payment is growing more popular these days, but it's still nowhere near being a dominant method of payment
Most people I know still don't use mobile payments on a daily basis
Momo is I think the most popular payment app in Vietnam, but as foreigners you won't be able to use it because you need to have a Vietnamese bank account