There are a few ways to go with this, depending on which vampire canon you go with:
1: Symbols of the One True Religion work, no matter who’s using them or who the vampire is, and nothing else does. Usually but not always, the author’s “One True Religion” is Christianity.
2: Symbols of any true religion work for everyone, and there’s more than one true religion.
3: All religious symbols work for everyone.
4: Religious symbols work if the vampire believes in them.
5: Religious symbols work if the person wielding them believes in them.
6: Religion has nothing to do with it; there are just some shapes that, for completely random reasons, happen to be effective, and a cross happens to be one of them.
7: Crosses or other holy symbols do bupkis, and that’s just propaganda spread by vampires or their agents, to make the cattle think they have a chance, and to dissuade them from anything that really does work.