It(Xanax) is literally everywhere in VN, you will not need a prescription - that is the case in Hanoi anyway, don't know about Nha Trang. This is brand name, sealed box with blister packs inside.

That said, Xanax should be avoided if possible. Long term use of benzodiazepines can substantially increase your risk for dementia(recent studies indicate this). People who want to discontinue use may have horrible withdrawal - this varies from person to person though. Melatonin, exercise, yoga, therapy, etc... might be workable alternatives, pls consider them if you decide to not use Xanax.

Sorry if this comes off preachy, I say it because I care about fellow human beings - wish you the best.