So I've been dealing with 2 vampires in my fort now, and have learned some things about how they behave, and how to keep them away from snoozing dwarves. Here are some tips for you to try if you can't figure out how to stop the vamp (these are tips for keeping them from draining dwarves, not for discovering the vamp in the first place).
1) Contain the vampire in a (sealed off or locked, or both) room.
2) When they are outside containment, NEVER LET THEM OUT OF YOUR SIGHT.
3) Assign them to burrows, they will stay inside them (afaik)
4) Locked doors keep them out of rooms, so if they start strolling to your sleeping area, lock the doors of rooms with sleeping dwarves, if you successfully do this (and the vamp was indeed heading for one of them) the vamp will instantly re-path and find another job.
5) Drafting the vamp and telling him/her to do something in the military will NOT discourage them from draining dwarves, they will readily disobey your orders and gorge themselves.
6) If you tell them to do something like pulling a lever, they WILL do it, and will not do anything else (even draining dwarves) until that lever is pulled (so theoretically, you could have a vamp burrowed to a lever specified to them that is set on repeat in the middle of your dormitory/rooms, and as long as the vamp doesn't go on break your dwarves are safe).
7) When a vampire wants to drain a dwarf, they take a fake job. This can include things like going on break, storing items, and the like. If you see this happen, you can check if it is indeed a fake job by going into the jobs screen (j), and trying to remove the job from the vampire (select job, press r), if the job does NOT get removed, it is definitely a fake job. Even if it isn't or you can't tell (like going on break), you should still take precautions and use tip 4.