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 革命機ヴァルヴレイヴ). Similarly, the forms ヸ, ヹ, and ヺ seem to be fairly rare too.
I can somewhat understand the rationale for using ヴィ and ヴェ for ヸ and ヹ given that ヰ and ヱ are almost obsolete in the contemporary written standard, but ワ is still a very common sound.
Is there a particular reason why the two-character form seems to be preferred over the single-character form?