He, along with several other turian mercenaries, was hired to work at the BAaT program in 2166, training human teenagers to harness their biotic abilities. The company which hired him, Conatix Industries, kept his involvement in the program secret: the memory of the First Contact War was still vivid in people's minds, and the fact human biotics were being instructed by turians would not have been looked upon favourably by the Alliance public.
Vyrnnus did not conceal his dislike of humans, introducing himself to one trainee by claiming "I was at the helm of the dreadnought that killed your father". His training regime was severe, involving the use of hunger and thirst as training aids. Although Vyrnnus did enable the trainees to use their talents, his methods took a heavy toll: several BAaT trainees "snapped" under the strain, and some even died as a result.
Finally, when Vyrnnus punished one of the trainees, Rahna, by breaking her arm, Kaidan Alenko attempted to defend her. Vyrnnus lost his temper and attacked Kaidan, claiming his people should have "bombed [humanity] back to the Stone Age". Finally, Vyrnnus drew a knife--a military-issue 'talon'--and Kaidan responded by delivering a powerful biotic kick to his face. However, Kaidan's retaliation was stronger than intended, breaking Vyrnnus' neck.
The staff at BAaT tried to give Vyrnnus medical attention, but he died before they were able to take him to the infirmary. Vyrnnus' death caused a diplomatic stir with the turians, which eventually resulted in BAaT being shut down.