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You would probably want to find a breakdown of what the most popular browsers (and versions) were for those respective language markets and consider those factors as well. Then of course it may depend on your niche market's users...but you all know that.
You'll find more information on this site: [alanwood.net...]
The major reason why we use UTF-8 on all our pages is this: when you have a mix of several languages (for example Vietnamese, Chinese, Korean and English) on the same Web page, all characters display properly.
That's why all (smart) translation bureaus use UTF-8.
Take a look at this article: A peek at Unicode's soft underbelly [www-106.ibm.com]
This came up in a recent discussion [webmasterworld.com] (msg#9) we had over in the Asia Pacific Forum. I get wary when people tout UTF-8 as the solution to encoding problems because I've heard a lot to the contrary. I'm really just playing devil's advocate here waiting for some of the old Unicode pros to show themselves.
Vietnamese is ~Latin a-z + alsorts of accents and diacritic marks which Unicode handles easily. That it is isn't a more complicated graphic script suggests Unicode will probably be the best for Viet also.
Of course the best way would be to ask a local Thai and Viet users who might be able to advise the prefered system in those countries.