Two-time defending Rogers Cup champion Novak Djokovic also moved on, cruising to a 6-2, 6-1 win against Florian Mayer.
Pospisil will next face 51st-ranked Radek Stepanek, who beat 12th-seeded Nicolas Almagro 6-3, 6-7 (4/7), 6-3.
Three other Canadians advanced, too: Milos Raonic, Filip Peliwo and Frank Dancevic.
The 13th-seeded Raonic used his booming serve to beat France's Jeremy Chardy 6-3, 4-6, 7-5 in a night match. He next plays Mikhail Youzhny.
Raonic said he felt no added pressure to keep the Canadian winning streak going.
"I'm very proud and happy for the guys that have been winning," he said. "But it doesn't affect myself personally.
The 19-year-old Peliwo, who won the boys' title at Wimbledon last year, staved off a match point in the second set with what he called a "lucky" shot, and then moved on when Jarkko Nieminen stopped playing in the third set because of a hamstring injury.
Peliwo will face Denis Istomin of Uzbekistan in the second round on Wednesday.
Dancevic defeated Lu Yen-hsun 5-7, 7-6 (8/6), 6-1 and next plays Wimbledon semi-finalist Jerzy Janowicz, who beat Julien Benneteau 3-6, 6-3, 7-5.
In the second round, Kei Nishikori of Japan rallied to beat Italy's Andreas Seppi 4-6, 7-5, 6-1.
Rafael Nadal, an 11-time Grand Slam champion who hasn't played since a first-round loss at Wimbledon, teamed with Pablo Andujar to fight off two match points and beat David Ferrer and Feliciano Lopez 6-7 (2/7), 6-4, 12-10 in doubles.
In other first round singles matches, 13th-seeded Fabio Fognini beat Marcos Baghdatis 1-6, 6-1, 6-1, Alex Bogolomov got past Michael Llodra 6-2, 4-6, 6-3, Mikhail Youzhny defeated Jurgen Melzer 6-4, 7-6 (7/3) and Nikolay Davydenko came back to beat Gilles Simon 2-6, 6-1, 6-1.
Results on the second day of the Montreal Masters on Tuesday (x denotes seed):
Novak Djokovic (SRB x1) bt Florian Mayer (GER) 6-2, 6-1
Kei Nishikori (JPN x9) bt Andreas Seppi (ITA) 4-6, 7-5, 6-1
Richard Gasquet (FRA x7) bt Martin Klizan (SVK) 6-3, 6-2
Radek Stepanek (CZE) bt Nicolas Almagro (ESP x12) 6-3, 6-7 (4/7), 6-3
Jerzy Janowicz (POL x15) bt Julien Benneteau (FRA) 3-6, 6-3, 7-5
Alex Bogomolov (RUS) bt Michael Llodra (FRA) 6-2, 4-6, 6-3
Filip Peliwo (CAN) bt Jarkko Nieminen (FIN) 3-6, 7-5, 3-1 retired
Fabio Fognini (ITA x13) bt Marcos Baghdatis (CYP) 1-6, 6-1, 6-1
Mikhail Youzhny (RUS) bt Jurgen Melzer (AUT) 6-4, 7-6 (7/3)
Frank Dancevic (CAN) bt Lu Yen-hsun (TPE) 5-7, 7-6 (8/6), 6-1
Vasek Pospisil (CAN) bt John Isner (USA) 5-7, 7-6 (7/5), 7-6 (7/4)
Alexandr Dolgopolov (UKR) bt Kevin Anderson (RSA) 7-6 (7/0), 6-4
Marcel Granollers (ESP) bt Grigor Dimitrov (BUL) 6-4, 6-4
Nikolay Davydenko (RUS) bt Gilles Simon (FRA x14) 2-6, 6-1, 6-1
Milos Raonic (CAN x11) bt Jeremy Chardy (FRA) 6-3, 4-6, 7-5
Tommy Haas (GER x10) bt David Goffin (BEL) 7-6 (7/4), 6-3