Dracula (I think) is known as an unnaturally convincing seducer. I remember something of that from Buffy as well. I suspect that may be why Vanessa's sixth sense didn't kick in when first dealing with Dr. Sweet as well, his mental power over people is that strong.

I was mesmerized about him as well. I kept wondering what his audition must have been like…"Could you slither for us around this coffin please?"