I backpacked around Thailand, Vietnam, and Laos for 3 months about 15 years ago.
For suits, the best prices were in Thailand at the time (Phuket, Patong Beach there is a great one, private message me if you want more info). I was surprised at how much the prices have changed as I get emails from them from time to time still and since they still have measurements that work for me, I have looked into some more from time to time. Back in 2010 I had 5 suits made and about 15 shirts, and a vest for each suit as well, and it all came in around $1000 CDN. I think you're looking at about $300 per suit and $50 per shirt these days so not sure it's worth in anymore in comparison to just going to Indochino or the like.
In Vietnam itself, as others have said, if you can afford it fly. We were on a budget and the trains and sleeper busses were absolutely horrible ways to travel. In those days, (it may have changed) they'd have locals literally sleeping on the floors of the busses, and same with the trains where you'd be in a private bunk but across the room their would be entire families taking up single bunks. Always overcrowded, hot and I'd avoid it if I could.
If you can, check out Hoi An. It was one of the cooler areas we visited. Na Trang was also cool as well as Ha Long Bay (we actually got rooms (with AC!) in a Vietnamese Junk Boat and sailed around the bay for 2 days).
You mention Thailand and my favorite places were Patong in Phuket, Ko Phi Phi, Kho Samui. If you have any specific questions about places don't hesitate to ask.
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