This idea is intriguing. Given the origins of the vampire story, though, I believe we are meant to accept that the holiness of the cross along with Christ's inherent protection is what wards off the vampire. This was the original explanation for the protection these items afforded, and I think Buffy the show continues this.
This always really bothered me! I understand that the origins of modern vampire lore is rooted in Christianity but there are plenty of other cultures and religions that had tales of creatures similar to the vampire that the show could have pulled from if they absolutely needed a religious angle every now and then.
Because a vampire is something against nature and against God then naturally something blessed in His name such as a cross or Holy Water should work to repell such a creature. But that only works if
1. You absolutely agree that such a God exists which the show does not
(CWDP “is there any word on that, by the way?" Answer: “Nothing solid.”)
2. This God is the same God depicted in Christianity.
If you don't follow these rules (which the show doesn't) then it makes no sense that a crucifix would do anything to stop a vampire.
If you think that the show kept these symbols to create an interesting juxtaposition of the unholy vampire then this could have been achieved using any religious symbols that would have invoked some sort of religious conscience or duty in the person while they were alive.
So then using a cross to repel Vampire Willow would not have worked but Maybe a Star of David or a Hamsa could have worked on her.
Star of David - this symbol is linked to the "Seal of Solomon", the ring used by King Solomon to control demons and spirits.
Hamsa - a representation of God's hand.
Setting reminder: // In Doppelgängland after Cordelia lets Vampire Willow out of the cage in the library Vampire Willow chases Cordelia and corners her in the restroom. Wesley, hearing Cordelia scream, intervenes by waving a cross in Vampire Willow's face and forces Vampire Willow to leave.//
And for a show that is so progressive (that even introduces many different Gods both fictional and rooted in real religions - Glory, Hecate, Osiris etc) it bothered me that they perpetuated the idea that
Christianity is the true religion because regardless of the religion of the person while they were alive a crucifix can be used to stop any vampire!
I understand that we can't see Buffy patrolling with a bunch of different religious icons in hand to see which ones would work on the particular vampire she is fighting at the moment (if any at all).
But maybe this particular "weapon" could have just been left out of the show all together. There are plenty more ways to hurt and/or kill a vampire!