According to the game's lore, Vassili became a freelancer after being fired from the government for missing a shot against his own sister. The sister was apparently running for some kind of office, which is why she was pinned for assassination. My guess is she was a target because she supported the use of mercenaries for battle in the irradiated London, and the British government most likely did not want to spend their money on expensive mercenaries, and would rather just use the military. The people who voted for her, who were also apparently joyous for their representative not getting killed, probably supported using no-name mercs with no familial ties to them instead of sending their loved ones to fight in an irradiated wasteland. The CDA probably picked her up because they liked the idea of the public knowing as little as possible about what's going on inside the wasteland walls, and using mercs that don't give a damn what the people want and are just working for money is the perfect way to do that. With that, Vassili's sister employed MercServ protocols into the CDA militia, and the events of Dirty Bomb would begin to unfold.