Knighton also earned an assist on Mattocks' key insurance goal in the 80th minute for the Whitecaps (8-5-5), who beat Seattle for the first time in league play in their third MLS season. Vancouver moved into fourth place in the Western Conference standings while extending its streak to five games.
The Sounders (7-6-3) saw their unbeaten streak end at four games.
Miller capitalized on Vancouver's first legitimate chance of the game in the fourth minute. He ran on to Corey Hertzog's long, high ball from the midfield line, sidestepped defender Jhon Kennedy Hurtado and put a shot just inside the far post.
Hertzog's assist was his first in 11 MLS games. He also has a goal to his credit.
The Whitecaps did not get another good chance again in the first half.
Instead, they had to rely on stellar goalkeeping from Knighton, who will be in a battle to keep his starting job after newly signed Danish goalie David Ousted becomes eligible to play once an international transfer window opens Tuesday.
Knighton was credited with five saves in the first half as Seattle recorded 10 shots. Three of them went wide and two were blocked.
Whitecaps coach Martin Rennie tried to generate some offense in the 76th minute as he subbed in striker Mattocks for Camilo, who was held in check most of the night and saw his scoring streak end at five games.
The move worked as Mattocks ran on to midfielder Gershon Koffie's long header off a Knighton goal kick and lifted the ball over Seattle goalkeeper Michael Gspurning. The goal was the first for Mattocks since May 11.