Much of the world uses mobile payment apps instead of credit or debit cards. Some banks allow a setting that using a card can require a ping to the banking app for verification of the transaction. I don’t know if it’s legal to turn down cash payments in Vietnam, but some vendors may only accept digital payments.

I guess you could take your laptop out at the restaurant and in the taxi to pay. It seems a little strange. You might better just use a browser on the smartphone instead of the mobile app.