Mass Effect: Foundation
Vyrnnus was serving at Jump Zero as an instructor for the humans' nascent biotic program, Biotic Acclimation and Temperance Training, in 2168. While his students were on meal break one day Vyrnnus strolled in their company and barked orders to assemble at the training room ASAP. The trainees scrambled to comply, and at the training room they found a pile of building materials and one of their own tied to a post.
After ordering the students into formation, Vyrnnus began lecturing them on the costs of their education: millions of credits were being spent on them, and apparently they didn't take their studies seriously. The boy tied to the post had rigged a transmitter to send messages off-station, an illicit activity under BAaT rules, and Vyrnnus fished for accomplices among the students. No one stepped forward, so the Commander devised a biotic exercise for them. The trainees were to biotically assemble an object from the pile of blocks on the floor, though if the assembly was wrong or a piece fell, the students would start over. Vyrnnus offered water for the trainees but only if they finished the exercise to his satisfaction.
Several hours later, students started dropping from exhaustion. Vyrnnus instructed the remainder to pay no heed to the fallen, but Kaidan Alenko managed to cover for Rahna when she faltered on her part. The Commander noticed the gesture and moved towards the pair, complimenting Kaidan on the catch. However, he began taunting Rahna with a glass of water as he noticed how tired and weak she was, and used his biotics to float the glass within her arm's reach. Rahna fell for the bait and the turian blasted her arm, calling it pathetic. Kaidan's rage boiled and unleashed copious amounts of biotic energy at the turian.
Vyrnnus remarked that Kaidan's got skill when he recovered from the blast, but added he'd teach the human what it meant to strike someone. Vyrnnus biotically lunged at him and Kaidan used his abilities to shield himself. Later, Vyrnnus drew his knife and goaded the human to finish what he started. Vyrnnus managed to slice at Kaidan's right side, but the human retaliated by blasting him again and delivering a biotically-charged jumpkick on his head before he could recover.
By 2183 Vyrnnus is just one of the characters from Kaidan's past, a past that may be shared with Commander Shepard. Vyrnnus, along with several other turian mercenaries, was hired to work at the BAaT program in 2166, training human teenagers to enhance 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.