Have you ever wondered where the idea came from that vampires don't have reflections? It kind of seems like a pretty random attribute for a monster to have. And I'm not talking about the sparkly vampires from Twilight, although they might not have a reflection, either. You'll have to ask Alison about that one.
In ancient times, people thought of their reflection as the physical manifestation of their soul. Some people even thought it was unlucky to look at your reflection because it makes it easier for an evil spirit to steal it. It makes sense now why a soulless creature wouldn't have a reflection then. Also, this is where the superstition came from that makes it unlucky to break a mirror.