If you arrive at a Tan Son Nhat International terminal, multiple SIM card vendors eagerly want to help sign you up to their telco provider. They can install and activate your SIM with all the requisite ID checks within 10 minutes.
The usual telco choices are Viettel, Mobiphone, and Vinaphone. We usually get Viettel, but the others are fine.
I suggest you get calls AND data as there are several instances where you might need to provide a number and receive calls (eg. food and grocery delivery, tour guides, calling accommodation, etc.)
Around 400,000 VND (16 USD) will get you LOADS of data daily for a month. Even 250,000 VND (10 USD) will normally provide at least a few GB daily.
If you are only staying a few days or traveling through multiple SE Asian countries, an eSIM might be a cheaper, more convenient option, and you will be connected as soon as you land.
Airalo has Vietnam and SE Asian eSIMs that you can download beforehand, provided your phone is compatible.
I have some 15% and 10% discount codes on the DEALS page.
If you have no idea what an eSIM is, take a look at this blog that explains what an eSIM is and how you can benefit from having one.