Update on my screwed-up vampire noble: i managed to make him go insane eventually. He was sitting on his 10-spear array, in 4/7 water and wouldn't move a step in any direction, so i let an engraver carve a fortification in the wall. The vampire was promptly washed through and _fortunately_ picked up the equally-bugged 'conduct meeting' job with the outpost liaison from way back when, else i would've had another citizen-draining rampage on my hands. He also finally noticed that 1. I had switched his military equipment setting to 'nothing, replace clothing' and 2. that he had no rooms to his name, while the old expedition leader has a decent office. He had remained stable at 'fine' to 'quite content' while i was systematically sabotaging his mandates. Stripping naked and registering the paucity of his rooms sent him crashing to miserable, two tantrums and a few weeks later, finally, insanity.
Since he's 'a nervous wreck',he went stark raving mad. Curiously, he didn't lose his 'on break' indicator and once i opened the passage so he could get to his observation tower, he instantly got moving, apparently hoping to pass the access door and drain all my citizens. Of course i forbade the door just before he reached it, but i strongly suspect insane vampires will not stop killing your people if they ended up stuck 'on break'. Now he's sitting next to the door and won't budge. I took a few ballista shots at him, but of course he's much too good at not getting hit for that to have any effect.