Well, if we treat incoming light as a quantum superposition:
|light⟩ = α|holy⟩ + β|unholy⟩
…and assume that vampires reflect only unholy light and absorb holy light, then anything directly part of the vampire’s “system” filters light this way.
So I guess the question becomes, “How does the filtering happen?” Is it by physical surface, or is there some kind of quantum holiness field that absorbs holy light nearby?
I don’t think the light “knew” from the beginning. The light started in a state of superposition, right? Both unholy and holy. Once it hits the vampire, only the unholy light is reflected, acting like a sort of filter similar to a polarizing lens.