Mirrors symbolize portals to other dimensions as they are used in divination (scrying) -a variation would be the crystal ball- and that made them very popular tools for magical practices. They are considered to be a doorway to the subconscious mind, to the soul (vampires have no soul, thus nothing is reflected when they gaze at their mirror image). Scrying would reveal information about a person, about self or future events. Alice in her second adventure "Through the Looking - Glass, and What Alice Found There", uses a mirror as a portal that leads her in a world of opposites, in a distorted reality. The same happens to Christine in the Phantom of the Opera, when she follows the phantom through a mirror. A dive into the subconscious mind, where the hidden aspects of ourselves lie.
A less popular symbolism of the mirror is that of reproduction. It makes a copy of the person or object reflected on it. Some say that if you see a mirror in your dream, it means the urge to reproduct. I wouldn't want to go too far into dream interpretation, as I believe it is different for every person, but I must admit I accept the power that universal symbols may have in the collective subconsciousness. Thus, I would be more inclined to believe that a mirror in a dream signifies aspects of the self rather than reproduction. But as I said, the symbolism of such a dream is up to you to decipher.