Bergeron appeared to put the Bruins in a good spot with his goal just 36 seconds into the second period providing a 2-1 lead. But Vancouver responded with four of the game's next five goals - two from old friend Loui Eriksson - to open up a 5-3 advantage and chase Halak (14 saves) from the game.
"It's the way that we didn't defend, the way that we didn't take care of the puck in our zone and we gave it right back to them," said Bergeron. "And obviously you'd like that as a forward in the O-zone, to get it right back, so we made it too easy for them."
Jake DeBrusk's second of the game pulled the Bruins within a goal with 2:42 to play in the second, but Erik Gudbransson answered with 34 seconds to go in the period to restore Vancouver's two-goal lead.
Boston had a golden opportunity to get back in the game with a 5-on-3 advantage midway through the third, but a shorthanded goal from Bo Horvat - off a turnover from Tuukka Rask - with 11:20 remaining effectively sealed the deal, giving the Canucks a 7-4 lead.
"You expect to score on those," said Cassidy. "We've been pretty good at that. Unfortunately, it didn't happen for us, like a lot of things. They were able to kill it. The shorty, again that's puck play that could've been avoided I think. We score there, who knows?
"[David] Krejci hits a crossbar at the end of the second period. It's 5-4 at the time, I think - it makes it 5-5. Instead we go down 6-4, so I think that was as big a turning point as anything during the game to be honest with you. Now you go into it tied going into the third, but it didn't work out that way."