I'd want to know where they were getting the blood, I'd get them tested for blood borne diseases, and then I'd get them in counseling.
I do love my child, and if she told me she was gay, or a vegitarian (so unnatural, LOL), I'd go with it.
But vampirism - if that is a word - is not normal, natural, or safe. When someone joins some cult activity, there is something else at play, and I think I owe to to her to find out what the problem is, and get to the bottom of it.
And the thing is, there are some groups that are really into this, and they drink each others blood. So don't just dismiss it, investigate.