Easiest way to get a working data SIM before arrival is to have a phone that support eSIM and a compatible provider
https://esimdb.com/vietnam
Honourable mention: 3HK DIY 365-days packages
https://www.three.com.hk/prepaid/DIY/en/offer/travel - am using the UK/US/Canada/Australia/NZ package as my primary data provider here in Canada - prices are in HKD
Thailand's AIS also offer their SIM2FLY package as a eSIM
https://www.ais.th/roaming/sim2fly/en/
These eSIM solutions do not provide ability to text (SMS/MMS) or voice calls