In 2399, the location of Vendor was labeled on a star chart in Fleet Admiral Kirsten Clancy's office at Starfleet Headquarters. This planet was located in or near territory claimed by the Romulan Free State. (PIC: "Maps and Legends")
This planet was only mentioned in dialogue.
In the novelization of "The Survivor", the Vendorian homeworld was consistently referred to as "Vendoria" and it was said that Romulans frequently visited the planet, during a period of several years prior to 2269.
According to Star Trek: Star Charts (p. 66, "United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography", p. 32; "Federation Historical Highlights, 2161-2385"), there was a star system named Vendor located in the Beta Quadrant. This was a binary system with a F-class star and an M-class star. According to a map of the Romulan Empire – prepared by the Stellar Archivist Lsel Essik, from the Romulan Master Data Catalog, in 2366 – Vendor was identified as a Romulan system. In 2378, the quarantine was still in effect.