Nicely plotted, but oddly undramatic. I think you have an epic story here that's begging for more detail, drama, and suspense. Of course, that'll mean tons more writing, so I sympathize! I would suggest maybe fine-tuning some of the dialogue to suit the characters better. I don't know if you're trying to sound like Rowling, but I dont' think Lupin would ever say something like "Yeah, all right." Snape is a little more in character.
Interesting concept. I am curious what Snape finds in the sequel. I liked the idea of the vamps having some respect for Lupin as the warewolf. The dialogue between Snape and Lupin was very entertaining.