Vaga was a planet near Pyron, in the Magna star system. It was unique in the system as it had two suns.
Vaga was the home world of the Vagans. They Pyrons devastated Vaga when they fired a plinium missile into its larger sun. The few Vagans that survived left in an inter-dimensional craft, designed with help from the Doctor. The Key of Vaga was the only surviving structure on the planet. The Doctor had designed it to allow the Vagans to return to the planet if they ever wished to do so.
The Fifth Doctor and Adric visited Vaga and were forced to delay the Vagan ship as its appearance alongside the TARDIS would have destroyed the planet. The Pyrons detected their presence and attacked with a space-shark. The Vagan ship used a beta-force field to prematurely detonate the craft's warheads, destroying it. The Vagans then left the planet, apparently for the last time. (PROSE: The Key of Vaga)