As it stands right now, holy water has an extremely binary interaction with vampires. If you're taken to processing for any reason, especially if you haven't managed to get much blood yet, your round is essentially over. There's always an Unga McBaldy seccie force feeding everyone who comes in at least 10u of the stuff, and when you're fed it's a 100% certainty indicator of your vampire status. There's no room for talking, no negotiating, no trial, you're perma'd.
This is a little unfun for the vampires, especially the ones who like to roleplay or try to stick to the shadows as best they can, and it kinda rail-roads the vampire into going on a robust rampage rather than maintaining cover for any length of time.
So my suggestion is fairly simple, and hopefully easy to implement: Holy water should make everyone throw up blood.
This would keep the effect of draining a vampire's power, which is critical if you're going to be jailing them, but it removes the instant 100% infallible testing for vampire status. This opens up the possibility for suspected vampires to be released from custody with good role-play and a little schmoozing if there's still plausible deniability. Sec might drain them and send them on their merry way. Inconvenient, but not round-ending. It would also add a factor of harm to non-vampire crew, hopefully making sec a little more hesitant to force feed literal mercury-water to every grey-tider that enters the brig.
I'm not an expert on balance, and I don't know if this would tip the scales critically in the vampire's favor, but I like roleplay more than I like robusting every schmuck who gets in my way and my hope is that this small change would encourage more roleplay between sec and vamps.