Holy water that has merely been blessed has no physics behind it, which means any power it has would be based on the deity that gave it power. In other words, for holy water to actually work, God would have to exist independently of people's faith in him. In other words, if blessing were the only component that made it work, then Holy Water's effectiveness on vampires could be used to prove God's existence. Or at least that the Priests were capable of wielding holy magic due to their faith.
I think a more realistic take is that if vampires exist, the holy water thing is actually myth and can't hurt them, though the other things can.