I handle Vietnamese on a daily basis in InDesign. There are ways to add additional languages to the Language dropdown, but I've never bothered. Likewise, I've never tried to install a Vietnamese Hunspell dictionary. We handle all editorial/spellcheck/etc. work outside of InDesign, so it'd be superfluous. (And I'd balk at the task of trying to maintain separate dictionaries for our two main Vietnamese populations - our work is often aimed at elderly Vietnamese populations resident in the US using a pre-1975 form of the language, but we also do work for recent arrivals, who use a post-1975 form of Vietnamese that is very, very different. See this page for commentary about differences in accent placement - which doesn't even begin to address the changes in terminology and word choice.)
In short, all I do is choose a font that supports Vietnamese, and mark the text as [No Language].
Thanks so much for your reply. This work will also be for an elderly Vietnamese population, and I did not even think to discuss this with the translator (but will now!!)
What types of basic fonts that support Vietnamese do you use?
My basic go-to fonts are all freebies that came with InDesign or the Creative Suite - my clients tend to be social service agencies without budgets to buy additional fonts. Minion Pro and Myriad Pro both have full Vietnamese support, in the versions that came with recent editions of the Creative Suite. (There may be a few weights of condensed Myriad that don't have Vietnamese glyphs.)
If you're working on Windows, I'd suggest using a free utility called BabelMap to see what fonts you already have installed that support Vietnamese glyphs. There's a handy "Font Coverage" utility where you just paste in some text, and BabelMap gives you a list of all of the installed fonts on your system that have the glyphs necessary to render that text.
ha ha. I think we are working with the same people :-). Thansk so much for your help and the tip about BabelMap!