Even though there is a katakana character ヷ, most of the time when I see something that is transcribed into Japanese as /va/, it is in the form ヴァ (say, for example, in the title of the anime series ...
In "Maiko Haaaan!!!", a bridge going over Yumekawa (a fictional river in Kyoto) apparently has "夢ノ橋" written on it, rather than "夢の橋". I had two theories about why that may be the case. One was that ...
Nowadays, katakana tends to be used for gairaigo and onomatopoeia, while hiragana tends to be used for native Japanese words. This is a slight simplification - more information is available here. ...