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 >.<
Reading FAQ and Forum Rules is very helpful ... so go read them if you haven't yet. Ex. - You could find out how to link mangas. Along with other useful things.
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!