This is great information @bkkmfj2648.
To further add to the above, the only place in Vietnam I've been to is Phu Quoc, and similar to Da Nang, the massage boys will make themselves known on the apps (I used Grindr), whether they are from a shop or freelance (although there is not a lot of choice there, and the grid even reaches to nearby Cambodia). One guy that approached me sent me a price list, from memory I think it was 300k VND if I came to his shop, or he said he could come to me and I can pay what I like. He ended up coming to my hotel and we had quite a good time. When I converted 300k VND to my home currency, it was incredibly cheap so I just gave him double.