Sennen no Yuki Not finished,but from what I remember it's what you're describing. ... There's another I ran into not that long ago that is complete , but I would have to do some digging around to find it.
________________ Yes yes, I know I make longwinded comments, but that's just me >.< I should proof read my comments more, but I won't... So keep in mind I'm filled with typos
Try Karin by Kagasaki Yuna it's "Helpful Vampire" though probably not in the way you are thinking, though it is still worth a shot. I really fell in love with the series, as well as Kagasaki's other works.
________________ Beware the Fox, for while he is wise, he is also mischievous and cunning!