The myths told through the ages tell us that Vampires are the undead manifestation of the human they used to be in life. Only the soul has been removed, (that which made the body human).
Since in the old days mirrors were thought to show the reflection of the soul, and a Vampire had no soul, it followed that they would have no reflection.
Of course when you think about it with common sense, it couldn't really be possible.
Even if the Vampire cast no reflection, what about their clothes? An undead dress or suit that also casts no reflection sounds more than slightly ridiculous, yet you never look into a mirror and see a suit of clothes walking around by itself, or pocket change just jangling through the air all alone do you?
Even if you say that the Vampire extends it's undead-ness outward beyond it's body limits where does it stop?
Also a good question for the ladies how would a female Vampire put on her make-up if she couldn't see what she was doing? That would go far beyond inconvenient and into the realms of the circus clown!

As we all know water reflects images too, so what would happen in a rainstorm, as the drops reflected and then landed would the Vampire appear and disappear like a flickering cine film?
As I said right at the start, when you think about it with common sense, it couldn't really be possible.

Lady C.