Just come here and do your research and get several treatment plans. Then choose one and ask for the letter, just like you would so to shop for any service. Then fly to Vietnam and apply for your DTV.
Understand that's how it works. Just seems like an unnecessary burden. I assume when one converts there is a fee. Not the first Thai process I've found odd!😅
Be cautious of excluded coverage. Almost anything cardiac, kidney, blood or liver related will most likely be excluded. Medications here are very inexpensive. About 30% of USA for equivalent generics. You'll might consider tucking the $500 a month you're likely to spend in premiums away and self insure.