It was even more insidious than that. It wasn't about self-preservation or selfish need... they were so brainwashed they considered it good to tell on others as a matter of course. One of the secondary characters is even proud of his own children for turning him in for talking in his sleep.
In some books, I'm thinking of Dresden Files, I think it has less to do with the faith of the vampire and more to do with the faith of the person using the symbol. With Dresden, he uses a silver pentacle that his mother gave him and it has the same effect.