actually, vampires aren't repelled by christian symbols, they're repelled by maths. the cross just happens to be close enough to a multiplication symbol to qualify.

this is unironically (kinda) the explanation they give in the Castlevania show

In Peter Watts' Blindsight from 2006, vampires are not mythical creatures but an ancient extinct subspecies of humans (Homo sapiens vampiris) revived through genetic engineering who serve as highly intelligent, sociopathic predators with specific adaptations like enhanced senses and a unique aversion to right angles (the "Crucifix Glitch," which causes a neural feedback loop like a seizure when they see right angles, explaining the folklore's aversion to crucifixes and geometric shapes...