Hi kingcasino and welcome to Tiki Community!
Ensure that you use the latest stable release (at least Tiki 5.1), since it has fixes to work with urf-8, and thus, it should work with vietnamese with no problem, afaik.
You have to set in all places requested by the upgrader (if you come from ealier versions from 5.x or earlier) that you set db and tables to utf-8, as well as convert content to utf-8.
Report back if any issues after that Upgrade (if needed).
I'm not usre if the documentation of Upgrades is updated, but if you have any furhter doubt, please ask again