I had this vampire. He has killed a few useless peasant and I didn't care because my population was too high, it was like 275 or so at the time. Well, he made a mistake and ate a legendary soldier for dinner. I got mad and convicted him of all crimes and put him in jail for 350 days. So I go on and forgot about him for a while.
Fast forward to when he gets released. I put the follow camera on him as the jailer comes to unshackle him. He proceeds strait to the bedroom level and seeks out another sleeping legendary soldier. Oh shit, red alert. I order all of my squads to station by his door, but the vampire sucks him dry and escapes before the military arrives. I am mad, but I decide to keep following him to see what vampires do.
They drink blood.
He must have been real thirsty, after 350 days of no blood, he had an insatiable desire for the stuff. He went door to door and sucked dry half a dozen dwarves in a row. I stopped being mad and started laughing. A child witnessed him in the act and they ended up accusing each other. I had so many dwarves go insane or berserk. I used my strong military to keep them in line.
I was on the verge of a spiral, but it never came. I lost maybe 20 dwarves over a few years time afterwards, but things balanced out eventually. At this point, I had put the vampire into the military and gotten him into legendary status too. I stopped letting him have his way when he murdered his own squadmate and sparring partner. That got me mad again. I convicted him of every murder and sent him back to jail. This time, for 1,500 days. There is still 1,300 days left on his sentence. I am eagerly waiting the day he is released and devours the entire fortress all at once. All 240 of them.