Knock knock!vampires infront if the door.

Yes sometimes a vampire will come and knock on the door and just stand there for a while. In those times, all they are wanting to do is visit. So if you invite them them, they will chat for a while and leave like any other sim. Those vampires who do come to drink your sims plasma will do so when they are all sleeping, and break in doing that creepy walk style. They will then come in uninvited, and put your sim under a trance, and drink their blood, leaving them passed out on the floor.

The vampires who broke into my sim's home, were never invited in the first place, which threw me for a loop, because all the old movies and TV shows always said a vampire needed to be invited in first, before they could enter.