Anton Blidh tied it 1-1 with his first of the season at 5:51, scoring on the rush from above the left face-off circle.
After his goal at 17:23 was overturned when Cassidy challenged for offside, Garland put the Canucks in front 2-1 at 12:31 of the second period with a wrist shot from the left circle.
Marchand tied it 2-2 at 8:45 of the third. Pastrnak's one-timer hit the post before Marchand found the loose puck after a scramble in front.
"I thought Pastrnak scored, so I kind of stopped playing for a second," Marchand said. "Then I saw [Bruins forward Nick Foligno] whacking away at it, and Bergeron. So I just tried to get close to it, and it was a very fortunate bounce to come out. It was nice to put that one away."
Blidh left the game at 15:23 of the third after being hit from behind by Canucks defenseman Oliver Ekman-Larsson four seconds earlier. Ekman-Larsson was called for boarding, and Pastrnak scored on the ensuing power play.
"No concussion or anything like that, but he hurt his upper body around his [shoulder]," Cassidy said of Blidh. "We'll reevaluate tomorrow. He's sore right now. He may miss time. He may not. That's something we'll have to look at."
NOTES: Bergeron ranks sixth in Bruins history with 219 multipoint games, and Marchand is ninth with 185. … Boston was 2-for-3 on the power play; Vancouver was 1-for-4.