I just got the Vampires pack recently and yesterday I had this issue as well. I was playing a human sim at the time and was at a park at night. I was not able to travel, go home, enter build mode, go to map view or save the game because a vampire was "breaking in". There was only one other sim on the lot at the time. I assume she was the vampire but it was hard to tell because she was just playing chess. Judging from her badly put together outfit, she was a randomly generated sim. After she finally left the lot I was able to travel and do all the other actions again.
Other posters have mentioned the Spice Festival. In my case I did not attend the Spice Festival (in fact I don't think I've ever attended the Spice Festival in this particular saved game) but this oddity did occur on the same day as I got a Spice Festival notice. My sim was in a club meeting at a spa at the time the notice about the Spice Festival popped up, but was no longer in a club meeting when the vampire glitch occurred.
I have played for many hours in a different saved game where one of my household sims is a vampire and have so far not encountered this issue in that game.
