Just some examples to illustrate the severity of this issue:

• Searching for “bác bỏ” (abandonment) takes you to “bắc bộ” (northern region), which redirects to “Bắc Bộ Việt Nam” (Northern Vietnam). If you’re a reader unfamiliar with MediaWiki, this may look like a political statement to you.
• Searching for “khóa học” (academic course) takes you to “khoa học” (science). If you’re a reader unfamiliar with ElasticSearch’s ~ operator, it seems impossible to use the search bar to find information on academic offerings at universities.
• Searching for “truyền thống” (tradition) takes you to “truyền thông” (communication). If you’re the same reader as above, it seems impossible to find information on traditions, and it’s kind of insulting that the site takes you to something random instead.

Of course, searching for “bác bỏ” wouldn’t take you to “Bắc Bộ Việt Nam” if the Vietnamese Wikipedia had an article on “bác bỏ”, but there are so many potential cases for confusion that the 40-some active editors cannot possibly write away the problem.

I’m considering working around this issue at the Vietnamese Wikipedia with a gadget that prepends ~ to any search from the search box that contains Vietnamese diacritics. But it’s a sledgehammer, and I’d much prefer to get proper language support into ElasticSearch or to turn diacritic folding off entirely.

Agreed, this is an ideal outcome with respect to wikis that have Vietnamese as the content language. Thank you! Case folding is probably unavoidably more aggressive at a non-Vietnamese Wiktionary, but Wiktionaries tend to include hatnotes or other navigation aids to homoglyphic titles anyways.