Alright. So, a whole lot of stuff happened all at once. I'm still very confused, but here's what I DO know for sure:
I denied entry to the vampire, at which point they immediately became hostile, then ran away in fear when a chicken pecked at its legs. It then ran around in circles in a small open room my miners were digging in, while said miners chased it around. The militia (who I had armed with crossbows) then came down and began firing at the vampire, joining the miners in chasing it around the fortress for about 10 minutes, with the vampire dodging almost every arrow, turning the area into a sci-fi battlefield of colorful lines flying everywhere, which I don't have pictures of because I'm dumb and forgot to take them.
(here's a page of the report. 16 pages of this, by the way.)This continued for quite a while, until one recruit, who was armed with a shortsword instead of a crossbow, woke up from sleeping, plodded downstairs, and stabbed the vampire in the stomach, killing it, before going back to bed and sleeping for half a season.
While of this was going on, there were about 3 trolls trying to get into the fortress through a staircase leading into a cavern; each of them broke down a door, and then were beaten to death by an unarmed farmer, who kicked their heads in. At the same time, the aforementioned performance troupe that joined the fortress apparently broke the pathfinding of all the dwarves in my tavern, who began spamming several hundred job cancellations per second.
I'm still trying to figure out what happened, but yeah. If anyone cares, I'll post any details that I find.