Dorms are a lot safer when vampires show up.
How? A vamp getting seen feeding does not equal a victim being saved, does it?
It does allows vampires to be jailed or accused if justice system's set up. Then you can forbid the chain so they'll never get away, at least that worked in 34.09 for regular prisoners.
Err. Here's one more reason why you might want to have bedrooms, not dormatories, whether your dwarves can see each other in them or not, and have enough bedrooms to gaurentee that your dwarves never have to nap on the floor, or go without a bedroom when they're ready to claim one.
Given ample free bedrooms, you can check any bedroom about 1 and a half months after each migrant wave and more or less snag any vamp you have at almost a glance.
Use q, select a. You now see a list of all your dwarves. Dwarves that have beds are brown, ones without are green. Tired dwares that can claim a bedroom will. If you dont assign bedrooms, the only dwarves with one had been tired. Ignore green children and babies, but check the thoughts of any brown adults. Normal dwarves will have drank by 1.5 months in your fortress. If that dwarf hasn't done any of this, it's a vamp. Conversely, if you see a LOT of green dwarves, you misjudged the time and need to wait a little bit longer.
But why chain a vamp? Why waste a useful and interesting part of your fortress? Again, by having enough beds to gaurentee that no one ever sleeps away from a bed, you have the choice to create a bunker for your vamps which does not include sleeping areas (bedrooms, dormitory, hospital) but does include everywhere and anywhere that you want the vamp to work. If the sleeping dwarves are outside of the vamp's burrow, then the vamp will faithfully ignore them, while doing everything you tell it to for as long as it can.
This method also protects your dwarves, without requiring one to die to give you a chance to catch the vamp at feeding and then chain it away.... that's a waste of two dwarves, both the food and the feeder.