SIM card!

Hi, I’m travelling to Vietnam to meet friends already in the country. Once I arrive I’ll need mobile data so I can order a Grab taxi and communicate with my friends to find them.

A lot of people online seem to recommend Viettel for when you arrive, is there anyway I can buy one of their sims before leaving the UK so I know I’ll have mobile data when I arrive? Thank you!

Edit: thanks for all your comments everyone - super helpful!