I remember before the Anita Blake series fell off the rails, the star of David not working on vampires because it was a racial symbol, not a religious one.

That makes sense to me, if you’re aiming for a “secular” explanation of vampirism. The true faith explanation still requires some source of supernatural power to affect the vampire from outside, while an amped-up placebo effect is sufficient to explain Matheson’s vamps. I always loved I am Legend for taking the idea so seriously!