I'm surprised it took this long for there to be a doujin about Sakuya possibly being a vampire hunter...

...or are there a few I'm missing?

Meiling's Manga : A Tani Takeshi's work

I remember another, but most of them are "old". Recently they don't take that part of Sakuya's theory.

This artist's old doujin from about 2 years ago?
And there's many more.

I'll have to keep a bookmark on this and read this when I get home. Origin stories are often interesting