The Fifth Doctor and Adric tried to stop the Vagan craft returning to Vaga as its presence there alongside the TARDIS would destroy the planet. The Vagan were already aware of this and arrived some time after. Using a beta-force field supplied by the Doctor when he helped build it, the Vagan craft prematurely detonated the Pyron space-shark's warheads, destroying it. Although they were opposed to violence, they though it fitting that the Pyrons were destroyed by their own weapons of destruction.
Valtar, a former friend of the Doctor's, used a hologram to talk to him and Adric, explaining how they were aware of their predicament and so had avoided landing. Following this, the Vagans left their planet, supposedly forever. (PROSE: The Key of Vaga)