At least 5 dwarves had gone missing, eventually found dead, drained of blood. There were never any witnesses, though I randomly accused people nearby the discovery of the crime to see how they'd react to being put in jail. Most of the victims were discovered in their own beds, but one was discovered in the hospital, where he had been waiting for treatment. Perhaps the vampire was attracted to the smell of his bleeding wounds.
At any rate, finally the vampire slipped, and killed in full view of two other dwarves. I got the notice that the body had been discovered, drained of blood, and viewed the crime on the Justice tab. There were 3 dwarves listed as witnesses. One of them, Reg Amuglibash, was randomly accusing a dwarf who wasn't even in the area of having committed the crime. The other two were blaming Reg Amuglibash.
I convicted Reg of the crime, and he was sentenced to ~200 days in prison. After he was chained, I walled off the prison to see what would happen.
So far, he has shown no signs of hunger, thirst, or sleep. His nickname "!VAMPIRE!" isn't displaying in the [v] interaction, nor in the unit list. I tried to recruit him into the military, but he doesn't show up on the list when assigning dwarves to a squad (is that because he's a vampire, or because he's in jail? I don't know.)
Meanwhile, I was sieged, and much of my military, despite being Lord-rank, was demolished. The rest of the fort is now slowly descending into a tantrum spiral, with citizens alternately going mad and going berserk. The vampire, walled off like a Cask of Amontillado, is listening to the screams outside his prison with no apparent emotion.