Jamieson, who also had a hat trick in the Chiefs' 8-3 victory in Game 4, almost single-handedly helped Six Nations grab a 3-2 lead in the best-of-seven series to decide Canada's senior lacrosse championship.
Steven Keogh had three goals of his own and Kedoh Hill scored twice and added two assists for the Chiefs, who can close out the series on Friday night.
Johnny Powless chipped in with two goals and an assist, while Colin Doyle had a goal and two assists.
Jeff Shattler had two goals and three assists for Victoria, while Rhys Duch added a goal and two assists. Cory Conway, Mitch Jones and Tory Gardner had a goal and an assist each, and Karsen Leung also scored.