Vietnam would be your llast option of the 3 choices.
I loved HCMC. But outcall FS is expensive. Hot tocs are fun but in most cases not FS. Plus the top tier providers may show disinterest by not making eye contact, playing with their phone or meal. Bike taxis are the cheapest in SE Asia here. There is going to be a language barrier here. And a lot of Viets will want to overcharge foreigners. Vietnam is better to bring a holiday girlfriend and sneak out for a massage. My friend with a bigger budget wasn't in love with HCMC. For your delicate budget it would be AC then Thailand. Your going to be limited to 2 shags a day max. Vietnam will have to be for another day because your P4P options will be strained on a shoe string budget.