I've rolled vampire a decent amount in the past few days, so take this with my entire shaker of salt:
Currently, there is no downside to force-feeding every single person who comes through processing an entire flask of holywater if anyone so much as mentions vampires on coms. They don't even have to use powers, or be suspected in any way for security/chaplain to say "Well make them chug anyway"
Suggestion: Make holy water toxic. Enough so that force-feeding a chug that would kill a vampire actually kills normal people as well. (The only difference is they don't burst into flames.) If you want a test for a vampire, you have to be careful. A dropper or syringe, and that's it.
Bonuses: Chaplain is discouraged from being an idiot with holy water, because it's dangerous. Additionally, antag chaplain has a way to produce a toxin to use on people. It would hopefully cut down on illegal executions as well.
Other notes: This doesn't actually help vampires a whole lot, because the testing will still occur - but at least there will be some reason not to metagame and just forcefeed everyone who gets arrested an entire flask.