Alejandro Silva scored the only goal of the game in the 58th minute, finishing with his left foot from the top of the box after a nice passing sequence from the Impact.
Montreal’s Anthony Jackson-Hamel also hit the woodwork twice on the evening, heading a Daniel Lovitz cross off the post in the first half and striking the bar on a curling effort in the second.
The ‘Caps weren’t without their chances in this one.
Anthony Blondell got into some dangerous areas in the early going, Cristian Techera sent a cross off the far post in the 33rd minute, and Nicolas Mezquida curled a 20-yard free kick just wide right early in the second half.
Whitecaps FC head coach Carl Robinson started a completely different XI after the weekend’s defeat at D.C. United, including a return to action for goalkeeper Stefan Marinovic and a first-team debut for Homegrown midfielder David Norman Jr.
Marinovic, who missed 10 straight matches with a knee sprain, looked his confident self between the posts. The New Zealand international was credited with three saves, including a diving stop on a Saphir Taïder header in the 63rd minute.
Norman Jr., who joined fellow midfielder Russell Teibert and left back Brett Levis as the three Canadians in Vancouver’s starting lineup, played 71 minutes in his debut and did not look out of place.
The ‘Caps will look to overturn the one-goal deficit next Wednesday, when they host Montreal in the second leg of the semifinal at BC Place (7 p.m. PT ). A ‘Caps victory by at least two goals would secure Vancouver’s berth in the tournament final, a 1-0 ‘Caps victory would send the match to extra time, and any other result would see Montreal advance.