So in theory this is true, but with how many people join and leave security (as well as Chaplains that just stand in processing to force feed water), the simple truth is it's going to keep happening. And when it happens and that person gets ahelped/bwoinked (and learns their lesson for next time), the vampire's round is still ruined even if they were careful in not using their powers or being discovered.
First thought: I'm totally on board for brain damage, if it's high enough to cause some of my favorite hilarious effects. (I will never forget the first time I saw someone bucklecuffed in processing being forcefed holy water and screaming "CAN U GIVE ME HALK?!")
Secondly: I was thinking on this, and I feel like it should be a numbers adjustment, so that using holywater should be a careful process, as opposed to CHUG CHUG CHUG CHUG.
Holy Water should be just toxic enough that 50u kills someone. 30u/40u should deconvert and put them at death's door, but not kill them. I mean this also gives the opportunity for smart security to keep carefully measured stock of 40u holy water pills on hand.