Vanden Prime was the main planet in the Vanden planetary system. This planet belonged to the Cardassian Union. In 2375, the planet was used by Damar and his Cardassian Rebellion as their headquarters. (DS9: "When It Rains...")
The name of this planet was not spoken in dialog but is taken from the script. In the script's list of sets the planet was called Avandi Prime.
According to the Star Trek: Star Charts, on page 46, this planet was located in the Vanden system. This system's primary was a red dwarf.