Corrosive Separation

Corrosive Separation. One of Tohno SHIKI's abilities as a demon hybrid. A curse of fusion by contact.
SHIKI is naturally capable of continuing to use his own body no matter what condition it's in, but his ultimate ability is actually restoring his own flesh by assimilating the flesh of others. In other words, it's the no holds barred version of an organ transplant.
Though SHIKI became a vampire due to being invaded by Roa's consciousness, this innate ability as a demon hybrid was already pretty vampire-ish. (Of course, in the end it's just making up for lost parts.)
The reason that Akiha attained some vampire-ish abilities after disposing of her rampaging older brother might have been because she also ended up plundering this power when she killed him.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised