In that case - I hope this maybe helps answer your question. If you open the Fontself Extension and then the font template, it shows a basic list of supported characters, but I’m not sure if every character you’ll need is in there.
You may also just want to look up a character map of Vietnamese and copy and paste the characters into a document somewhere to have them handy (if they’re not on the advanced list). In the app, you have to individually label each glyph, and I found copying and pasting from one source to be the most effective way. I made myself a master list of characters I was using in an Illustrator document so I didn’t have to type them out/search for them every time. It came in handy for testing my font as well, because it would clearly highlight which characters I was missing (they’d appear as the original font style).
Someone in another forum created this character list which looks to be complete for Fontself but maybe not what you’re looking for.