Stumpel scored his fifth goal of the season on a two-on-one with Jokinen.
"He made a really nice play and I had a wide open net," said Stumpel, who was crosschecked in the head by Koivu at game's end.
McLean made it a two-goal lead four minutes later at 18:12 when his shot from the right point beat Canadiens defenseman Mike Komisarek, who was left trying to play goalie after Carey Price's aggressive puckhandling led directly to a costly blunder.
"I was just trying to help my D-man out and tried to move it as quick as possible and it wound up on the wrong stick," Price said. "I'm not going to change my game at all. It's something that I can utilize and I just made a mistake. It's part of hockey."
Stephen Weiss increased the Panthers' lead to 3-0 when he beat Price from the left side 7:05 into the third.
"I got a piece of it, I just misread it a little bit," Price said. "He came down the wing with a slap shot. It's hard to read a slap shot. It didn't have as much velocity as I thought."
Vokoun had two shutouts in his previous five games and lost a bid to add another 8:25 into the third when Andrei Markov scored.
Markov got his second goal of the period with 23.7 seconds remaining with Price on the bench for an extra attacker.
Florida held on despite a flurry in the final seconds for its fourth win in six games (4-1-1).
"Everybody's trying to push and get closer and score a goal, especially a home game, they're just not going to give it to you and sit back and say, 'Oh well, you win tonight,'" Vokoun said.
Koivu, Montreal's captain, drew 15 minutes in penalties for crosschecking Stumpel as time expired.
"I just tried to keep him on the boards and then I fell down, and then he came from behind and just crosschecked me right behind the head," Stumpel said. "It was pretty much over. You don't expect it. Maybe he was [upset] because the game didn't go his way."
Koivu was unaware after the game that he had been assessed a major and a game misconduct for the incident.
"What was the call? I don't know, for what?" Koivu told reporters when informed of the penalties.
"We tried to look at it on the video but we couldn't see it, the camera moved away," Canadiens coach Guy Carbonneau said. "I don't know what happened."
Montreal has lost seven of eight at the Bell Centre (1-5-2).
Jokinen set up Florida's first goal when he poked the puck past Komisarek along the left boards to set up a two-on-one.
The Finnish center drove into the offensive zone before passing across to Stumpel, who beat Price with a shot into an open right side.
The rookie goalie had nobody to blame but himself after McLean made it 2-0 later in the period.
Price rushed to his left to cut off a puck in the corner before it crossed the red line. He immediately fired a hard forward pass -- directly to McLean, who put a shot from just inside the blue line into a wide open net, despite Komisarek's game attempt to fill Price's role.
"There's a time for everything, and he could have reacted a little faster," Carbonneau said. "We talk to our defensemen and our forwards and tell them not to make plays through the middle of the ice. If he would have kept it on the boards, there would be no goal."
Vokoun, chosen 226th overall by Montreal in the 1994 entry draft, had shutouts in a 1-0 win in St. Louis on Thursday and a 3-0 win over the New York Islanders on Dec. 7.
Price made his seventh straight start and stopped 27 shots.
"They play a shut down game and they're pretty good at it, and I think they did a really good job tonight," Price said.
Cristobal Huet, cleared Saturday to return from a groin injury that sidelined him for five games, backed him up for a second straight game.
The Panthers are the only NHL team with a winning record against Montreal. Florida, which won its two games at the Bell Centre this season, is 25-16-6-4 overall against the Canadiens. ... Montreal left winger Guillaume Latendresse was a healthy scratch. He was out of the lineup for a second straight game after suffering an upper body injury in Thursday's 4-1 win in Philadelphia.