When your world allows psychic powers (for which you can often find a pseudo-scientific explanation), you could say that vampires are actually completely invisible and their physical appearance is just a hallucination induced into the observers mind through telepathy. Whatever mechanism is responsible for causing this hallucination is not smart enough to take reflective surfaces into account. It might or might not also manage shadows. When vampires can control this effect consciously, it would also give them the ability to change their appearance at will or suppress it to become invisible. Transformation and invisibility are common ability of vampires in many vampire stories. However, the end result would be that your vampires would be more like ghosts than like living corpses.