Michal Holub had a goal and an assist for the Thunderbirds (4-3-0) as Tyler Alos and Seth Swenson added a goal apiece.
Riley Sheen and Roberts Lipsbergs each had two assists for Seattle.
Cain Franson led the Giants (2-6-0) offensively with two goals while David Musil and Marek Tvrdon scored the others.
Brandon Glover made 28 saves for the win as Tyler Fuhr allowed five goals on 14 shots before getting pulled in the second period.
Liam Liston stopped 6-of-7 shots in relief.
Seattle went 2-for-6 on the power play while Vancouver scored on their only power-play opportunity.