Mixing religion in with vampires really points out the fictional nature of all religions. They're just as goofy as vampires.

Bless

Level: 1st Casting Time: 1 Action Range/Area: 30 ft. Components: V, S, M * Duration: Concentration 1 Minute School: Enchantment Attack/Save: None Damage/Effect: Buff

You bless up to three creatures of your choice within range. Whenever a target makes an attack roll or a saving throw before the spell ends, the target can roll a d4 and add the number rolled to the attack roll or saving throw.

Well the answer seems simple. The clouds neither count as creatures, nor are they in range.

Cleric: Mind your manners, scum. Don't forget how we dealt with your kind in Aafrika.

Vampire: YOU KILLED HUNDREDS OF MY KIND, YOU MONSTER!!!!!

Bard: Um, what the heck did you do?

Cleric: I blessed the rains down in Aafrika.

I mean, they could do it if they're many, as there's nothing that a hundred men or more could ever do

Because the priest uses his FP to infuse the water with Holy +1. But the amount of FP is fixed, based on the current level the priest has. This means the more water the priest tries to infuse, the weaker the effect. When infusing a small vial of water, it's very effective. For a vampire it would be like throwing scalding hot water at a regular person, burning them badly. But if you try to do something larger, it doesn't work as well. Once you get to the size of a coke bottle, it would be like just luke-warm water, not doing a lot. At that point, you'd be better off throwing pocket sand.

The duality of woman

Holy water works only for christian vampires. Atheist vampires remain unaffected.

It’s like antibiotics. If you keep blessing water without restraint, soon you’ll get holy-water-resistant vampires.

My toilet water is already holy. It’s filled with spirits every night.

Could just bless the water in their bodies.