Whitecaps finished in second place in the Western Conference above Timbers on goal difference at the end of the regular season, and after a 0-0 draw in the first leg they went on the front foot from the off in front of their own fans, with Kekuta Manneh threatening early on.
The winger had the first opportunity of the game when he collected a throw-in, swivelled, and fired narrowly wide of the far post, and he came even closer minutes later when he ended a fine run by drilling against the post, with the ball bouncing off Portland goalkeeper Adam Kwarasey and bouncing inches wide.
However, the 20-year-old livewire had his evening cut short before the half hour when he had to be replaced after picking up an injury, and that was the catalyst for Timbers wrestling control of the tie.
It was the visitors' top scorer Adi who broke the deadlock with an explosive strike just five minutes after that substitution, when Diego Valeri was played in behind and laid the ball back for the striker to smash home into the roof of the net from 10 yards and leave Whitecaps needing to score twice.
Valeri's swerving effort from distance had to be saved by David Ousted as the away side looked to extend their advantage with a second away goal, biut Whitecaps had a sniff of an equaliser just before the break when their own top scorer Octavio Rivero glanced a header wide.
Ousted again saved another tricky Valeri effort before Rivero's diving header was blocked by Alvas Powell as both sides had chances early in the second half, and the home fans were screaming for a penalty when they thought that Rivero had been brought down inside the box.
In truth, though, Portland had control of the match and only a fine recovery save from Ousted denied Valeri's whipped free kick from creeping in and putting the tie to bed.
Vancouver substitute Robert Earnshaw came close with a late overhead kick attempt, but Adi played in Diego Chara to seal the win on the counter-attack deep into injury time and ensure that Timbers set up a date with Dallas after the Western Conference winners beat Seattle on penalties earlier on Sunday.