So what do the traps do, exactly? Cause they seem like they're deadly, but then later the vampires purposefully drop a girl in there. Is it some sort of preservation system then?
Supposedly that's how they work, yes, since they were originally designed to trap people for the augers to eat later. It's a system of the vampires' design that has been subverted by the good guys. However, a couple of the traps don't seem to suck their victim in, drop them down a chute, or what have you, merely catapulting them away like the one activated from outside the house. How those were supposed to have been any help capturing people isn't exactly clear.
Perhaps the vampires believe the traps will leave the victims with a broken leg (or other injury) that will make escape impossible.
It's also possible that those latter traps, the ones that catapult people away instead of dropping them down a trap door, weren't planted by the vampires at all. SCAT managed to get control of the override system, after all; who's to say they didn't plant a few traps of their own in case of emergency, like having to protect the present girls from whoever or whatever made the previous girls disappear?