Vampires

Yes, vampires have existed longer than the Christian faith. The idea is someone with a high faith stat imbues a volume of water with holy energy. The higher the stat, the more energy gets imbued. This however does rely on what ritual is used and how well it's performed and of course the stamina and magic level of the person performing the ritual. The holy energy transferred also drops off with time, this is a logarithmic curve. So a high amount of energy drops off fast, where a lower amount of energy can stick around for a while.

The volume of holy water flasks isn't just arbitrary chosen, it's the exact right volume for a common priest to imbue with holy energy and have it stick for a practical amount of time. Now remember the holy energy doesn't really do anything to vampires, sure it burns and hurts them, but they are eternal beings not pussies. It won't cause real physical damage, at least not any meaningful amount. So it's best use as something akin to pocket sand, something to distract and annoy vampires with.

Some monsters other than vampires are susceptible to holy damage, but many aren't. So the practical use is limited. In theory you could get a bunch of priests together and have them bless like a firetruck full of water. But no group of vampires would be caught out in the open like that. They employ guerrilla tactics, not open warfare. Plus the rituals will get in the way of each other, so the energy transfer is actually very poor.

Honestly this is basic stuff, the local library has many books on this very subject. Ask your local librarian for something like Hal's tombes of magic part 2 and 3 or Abbacula's collected works on Divine Magic, it's all in there.

Or like a wise person once said: You'd be surprised what a shoulder mounted rocket launcher can kill.