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. Any clothings or accessories which are part of the vampires projected self-image would not show up in a mirror, but anything they picked up from the environment would.
When vampires can control this effect consciously, it would also give them the ability to change their appearance at will or become invisible. Shape-shifting and invisibility are common abilities of vampires in many vampire stories.
However, the end result would be that your vampires would be more like ghosts than like living corpses.