What stops said Vietnamese for working the same 5 hours a month remotely?

Mostly, that it's not worth your bosses' time to micromanage you if they are going to extract merely 5 hrs/month worth of labor out of you.

But nothing prevents you from working 50 hrs/week for one decade and then retire early, move to Vietnam, and live from your savings.

I say 5h/month but you can always do a freelance contract full-time for a month and then chill for four months and repeat.

The point stands, if you're an American programmer and think you have to work full-time to literally just survive you're wrong.

Nothing, if they spoke English and had programming skills.

But that's not a small if.

And not just english, but they have to communicate well, be on the right time zone, and understand the culture as well. It's a very very rare find.