The Mansion Of Mirrors is a location in the Smurf Forest that Vanity had visited in the cartoon show episode "A Maze Of Mirrors". The house is so named because of all the mirrors that lined its rooms and hallways, which have the ability to capture the reflection of whoever looks into it and generate a living copy of that person from the reflection. However, with the multiple reflections coming to life, the original person who cast the reflections will vanish completely.
In the episode, Vanity generated multiple copies of himself by looking into the mirrors in the mansion, which created a lot of confusion and hurt feelings among the Smurfs as they dealt with so many different copies of Vanity. Eventually Papa Smurf found out what Vanity did and had him go into the mansion with his reflections while carrying a tuning fork coated with a magic formula, telling him to strike the tuning fork in order to break the mirrors and thus reverse the spell that was causing him to vanish. Although Vanity struggled with whether he should break the mirrors in his mansion, he eventually did what was right and thus restored himself to normal as all his reflections vanished.
The same place is called "The House Of A Thousand Mirrors" in the Smurfs comic book adaptation.