Here's my logic:
There is a possibility that she is a Hippo Leaguer- vampires who don't drink blood are kind of obsessive and erratic about things, and I can see Isaac using that as justification.*
There is a possibility that she is a bloodthirsty vampire, who will eat us.
Now, there is also a possibility that she is NEITHER of these things. If that's the case, it means that she is a Black Ribbon vampire who may or may not be from Ankh Morpork.
Perhaps a character claim is in order?
*If this seems like reaching to you, I'll just say that there aren't really that many strident anti-foreigner characters in Ankh-Morpork. They tend to be like, "Oh yeah, we hate Klatchians... but not Mr. Abdul down the street, he's great! And not the tourists, because I can sell them overpriced snowglobes."