When I worked in Viet, the bike to own was the Russian 125cc Минск 'Minsk'.
They were good for about 90km flat chat which is way faster than you'd want to go on a Vietnamese road. The engines didn't last long but you could get a top end done for $25 and a total rebuild for less than $100. If they used original Russian parts the engine would go for ages but there were some really crap Chinese knockoff parts that would last a week.
The bikes could be had for about $200 and I owned 4 of them. The first was a really tricked up cafe racer look but it attracted too much attention.
One ride I was on, I was in the countryside and an old Viet dude was admiring it, he was on a totally stock Ag model that looked in great condition, so I asked if he wanted to swap and he did. He loved the one I traded as it was so full of bling and looked pretty cool.
The one I got looked exactly like this.

Then I picked up what they called the sport model which had better suspension and travel and slightly bigger so I could stretch out, so it was good for extended bike trips.