Does "God" exist in the Buffyverse?

[EVIL UU]

People in places like India and China (which between them have half of the world's population but not a lot in the way of crosses and holy water) must have some similar tools, or all the world's vampires would congregate in those areas for easy pickings.

Actually, the concept of holy water is pretty-much ubiquitous, it exists in Indian and Chinese religions as well.

but people managed to fight them.

No, people did not manage to fight them. Whenever vampires became a significant threat a Slayer was called in the endangered area and thrown at the problem. The system worked up to India Cohen. And vampires managed to survive for so long only because a bunch of worthless wannabes kept their existence hidden. At that point humanity would be able to drive those vile fiends extinct, with holy water or without it.

 

[Xin Rong]

Actually, the concept of holy water is pretty-much ubiquitous, it exists in Indian and Chinese religions as well. .

This corresponds to my personal view. The holy water has an effect because people believe it is holy, and therefore the antithesis of the evil present within vampires. Therefore the source of the holy water isn't a neccesity... so long as it spans from a pro good religious belief then it affects vampires. The water is just an embodiment of peoples faith in goodness so repels vampires.