Sorry if this is already evident to you...but the differences between Wikipedia sites is not just about the language. vi.wikipedia.org is independently edited and maintained from en.wikipedia.org. The Vietnamese wikipedia page [1] is not as lengthy as the English one. And I'm not as good as reading Vietnamese as I should be, but the Vietnamese page doesn't seem to have the same content for "Media and censorship" that the English one does...among other differences. So I don't think it's as simple as "Send them to whatever language they used" (though I guess this is one case where seeing Google's source code would be enlightening, so...touché?)

[1] https://vi.wikipedia.org/wiki/Việt_Nam

I have no doubt that the English page is higher quality than the Vietnamese one. That won't matter at all unless the person searching can read English. When looking for information, there are no concerns greater than "even if I find it, can I understand it?"

Even if the Vietnamese page were better, there's no excuse for replying to an English query with a Vietnamese result.

Additionally, it could depend on whether they are using google.com or google.vn.