Vexatious Vermin is a solo mission you can take after Sheila is taken into quarantine. Once you pick the request up from Thanos, head for the Vareth Magic Institute to speak with Roche at the front desk. After a cutscene, Daniel joins your party and the battle with the rats starts. After the first quick battle, another cutscene starts where Jack and Daniel chase a Tawny Rat into the sewers for another battle with more rats. After the battle, the Tawny Rat gets away and Jack splits the pay with Daniel.