"vu" produces 「う゛」 instead of 「ゔ」
Inputting "vu" in hiragana produces 「う゛」 HIRAGANA LETTER U + KATAKANA-HIRAGANA VOICED SOUND MARK, instead of the more correct (and better-looking) 「ゔ」 HIRAGANA LETTER VU. Not a major problem since 「ゔ」 isn't often used in hiragana, but still a bit inconsistent.
If I understand the code correctly, this would need minor changes in libkkc/rom-kana-utils.vala (line 71) and several data/rules/*/rom-kana/default.json files.
I could probably make a pull request, but I have no idea how those work.
I agree with you about this change. The only reason why we don't have ゔ is that it is not included in JIS X 0208.
Ah, that's a rather good reason. I guess I'll just add "zvu": ["", "ゔ"], to my personal config, then.