Vancouver kept a pulse in its playoff hopes with an impressive victory at high-flying Seattle.
Manneh took advantage of some poor defending and scored in the 12th, 42nd and 54th minutes, notching the Whitecaps' first hat trick since entering the MLS in 2011.
The loss was Seattle's first at home since dropping the season opener to Montreal back in March and could be a significant blow to the Sounders' chances of winning the Supporters' Shield for most regular-season points in the league.
It also prevented Seattle clinching the Cascadia Cup for the regional rivalry between the Sounders, Whitecaps and Portland Timbers.
Seattle must win Sunday at Portland to claim regional bragging rights, otherwise it'll go to Vancouver.
The scoreless draw between Houston and Sporting Kansas City extended the Dynamo's unbeaten streak to five games.