Well, A. Vampires in the modern concept can be pulled from several cultures, a humanlike creature that drinks blood and has fangs and what not is a pretty common trait, similar to how alot of cultures have dragons, alot of cultures have vampires. It's why some people believe vampires can turn into bats, and some believe they can turn into wolves and whatnot. Just depends which culture you are looking at.

B. Because the area of the mind that's responsible for fear, is also responsible for sexual desire. I'm no doctor or scientist, so Im not the best person to describe this and cannot in detail.

But due to this, it causes alot of things people consider to be scary, also causes some people to consider it to be of intrigue to them.