Vanden Prime was the prime planet in its star system. This planet was claimed by the Cardassian Union. In 2375, the planet was used by Damar and his Cardassian Rebellion as their headquarters. (DS9: "When It Rains...")
The base, along with seventeen other rebel installations, was located and destroyed by the Dominion, weeks later. (DS9: "The Dogs of War")
The name of this planet is not spoken in dialogue but is taken from the final draft script of "When It Rains…". In the first draft of the script and even in the final draft's list of sets, the planet was called "Avandi Prime". [1]
The first draft teleplay for "When It Rains…" additionally established that this planet was located near the Likarian asteroid belt and that the Cardassian Rebellion's base was situated "in a cavern on the tip of the southern continent."
According to Star Trek: Star Charts (p. 46) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Vanden system was located in the Alpha Quadrant. The system's primary was an M-class star.