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?