Can you pay for Grab taxi in Vietnam with cash?

Yes, of course you can. Immediately after installing the app and registering, this payment method will be available by default, and if you want to add a card for payment, you can do so in the account settings. But Russian cards do not work there, including UnionPay, so if you don't have a foreign card, you need to pay for the trip in cash at the end.

More about the Grab taxi app in Vietnam is described here - https://nashaplaneta.net/asia/vietnam/art-chto-takoe-grab-taksi-i-kak-im-polzovatsa

How much does a visa to Vietnam cost?
Where to go in Vietnam for the first time?
Where is best to buy tours to Vietnam?
Follow the link, all these questions are discussed in the chat - https://t.me/vietnam_travel_chat