It’s something I’ve noticed in a lot of movies lately where there are vampires; and they are the main villain. Holy water, garlic, wooden stakes, sunlight all work like a charm; but for some reason, when the author/director wants to show HIS vampire is really dangerous, they will make it immune to the warding power of the crucifix.
Now, what does not make sense is that these same vampires usually are still repelled by garlic, if the characters remember to use it, and tend still to be harmed by holy water. So, the vampire can’t resist being warded off, and can’t resist holy energy; yet somehow it is immune to the crucifix which wards using holy energy. Logically it should work TWICE as well.
The only movie I’ve seen which makes any attempt to explain this is actually the new Fright Night. Not exactly the movie I expected to fix this lore, but I like it’s explanation. Early on, someone uses the crucifix against him, and his response is more or less ‘you don’t have enough faith’
You see, most Crucifixes are not actually blessed; they are mass produced and sold in stores. So, your faith would be the only source of holy energy behind the symbol. Of course, Van Hellsing has no real excuse here; as he had a tag-along priest who could have blessed the crucifix for him. I can’t remember though if holy water worked or not in that movie.
If we used this line of thought, using a small amount of holy water on a crucifix would also empower the crucifix to ward off a vampire. just be sure to use a small amount only; you want to save some holy water in case you need to toss some in his face.