Vega IX is the ninth planet orbiting the star Vega.
Vega IX was once home to the Vegan species, who ruled a large area of local space until at least the 20th century. After their extinction, the dominant lifeform was the apelike scora. (ST reference: The Worlds of the Federation) The planet was later colonized the Federation, as the site of the Vega colony, as well as by Delta III. (TOS - Star Trek 2 novelization: Tomorrow is Yesterday)
On April 4, 2163, a meeting made to finalize the trade agreement between Sauria and the United Federation of Planets took place at the Vega colony between Federation President Thomas Vanderbilt, Presider Moxat of the Global League of Lyaksti'kton, and the Basileus of M'Tezir. The Basileus attempted to get Captain Bryce Shumar of the USS Essex to ask President Vanderbilt to provide the Kingdom of M'Tezir with the resources to conquer all of Sauria in exchange for a monopoly in trading for the wealth of rare metals under M'Tezir, such as dilithium and trilithium. (ENT - Rise of the Federation novel: A Choice of Futures)
Montgomery Scott visited the planet sometime during his long career, and disliked many things about it, including the food. Unfortunately for him, the food was very popular with the Kropaslin. (SCE eBook: The Future Begins)
In 2409, the planet was invaded by the Borg Collective. A Miranda-class frigate on a cadet training cruise was the only Starfleet ship in range and conducted the rapid evacuation until reinforcements arrived trying to repel the fleet of Borg cubes. Starfleet was unsuccessful and quarantined the system, and Vega IX fell to the Borg. Casualties included Captain Va'Kel Shon's wife and children. (STO mission: "Assimilation of the Innocent"; STO short story: "Mirror Image")