CW: Animal cruelty (even though it's technically not an animal but a vampire, you get it.
A vampire turning into a bat is so intrinsically vulnerable and whumpy that I'm surprised I haven't seen this exploited in any vampire whump story out there (mine included).
Like of course a bat is tiny, cute, and extremely fragile, quite the opposite of a vampire. But the context!
Maybe silver triggers the metamorphosis, and they can't shift back as long as it touches them. Clip a little silver piercing through a membrane, let the vampire go, and you have effectively made a vampire harmless ! Nearly ethically too !
Maybe there's a delay between metamorphoses. Like they can't spam shifts, they must wait. So while they're stuck as a bat, they're vulnerable.
Maybe they can shift as will, but they're made to shift under a threat (a hunter menacing them with a stake), or they shift to try to escape, and they're locked into a cage, small but solid enough that if they try to shift back, they'll be crushed by the silver bars.
Maybe the whumper doesn't even know they're a vampire. They just smacked a bat and now it's crawling and trembling pathetically on their floor while a human loom over them. Injuries made to their bat shape translate to their vampire shape.
In rural areas in Europe, recently enough (now bats are protected so I don't think it happens anymore) bats used to be hunted down and nailed to barn doors, alive. Bad omens and superstitions yk.
A very old vampire surfacing from slumber for the first time in a while, shifting to explore, and having their bat-senses scrambled up by the technology.
Also great caretaking opportunity !
Of course the good old "Oh an injured bat on the sidewalk ! Let me put it in a dark box and bring it home to nurse it back to health!" And a crashing sound in the middle of the night as, surprise surprise, a bleeding vampire appears in the box.
Vampire in bat form holding onto caretaker's shirt, just snuggling there under the jacket. Only place they feel safe.
Also if you do know whumpy stories that uses this plot, or if you plan to write one after these ideas, please share ! It's just so good.