That's the thing. He wasn't exactly born a vampire. He did have vampiric ancestry from birth, but he got turned into a vampire because of science. Either way, he's not usually in his vamp-mode, so it's possible that he just might not be able to tap into that side of his ancestry when she's around.

Awesome. :D If you want a basic rundown on how things in Kuja's canon go let me know.