They can't be near any religious objects. Remember Dracula? He was just your average Joe until the Cinta of his life dies and when he went to a chruch, he swore to God so as a punishment, God transformed him into what he is now till he learnt his lesson. THAT'S why.
It is sad and creepy. I think there are a couple of buku out there about vampires. Check out your local perpustakaan and/or book store for them and anda get your desired infomation.