I would guess it would depend on your budget. The "cheap" suits I see that are decent quality are still actually pretty expensive by most standards ($800), and from what I've read they tend to charge more if you're not a local (people citing $1,500 or so). That's in Hong Kong, but I would imagine that it's not much different (and that's not really all that much different from lower-end US prices).
You get what you pay for. People with real skill aren't going to crazy-undersell themselves even if they happen to live in Vietnam.
When it comes to cosplay it's better to have someone that specialises in it - not a random seamstress or tailor who has spent many years doing something else. For instance, making suits is very specialised - you would not go to those people for most cosplay unless it was a fairly normal suit, even if it was cheap. You need a person with an eye for extreme detail - and that's not usually what those people are in business to do. And a lot of cosplays aren't JUST sewing and lots require special shaping and also sculpted pieces and other weird stuff - and you need a person that understands the process of cosplay from looking at reference images to picking fabric based on them and more - your average seamstress for hire doesn't usually have to deal with that kind of thing, especially not on the scale of cosplay.
Maybe there are people if there is a huge market for cosplay in Vietnam, but a quick Google search shows that is probably not the case.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan