Fleury turned aside all 25 shots he saw over the first two periods before exiting the contest. He was knocked over by teammate Tyler Kennedy in the second period as Kennedy was pursuing a Montreal attacker near the goal.
Fleury finished out the period but Vokoun came in to start the third, and responded by stopping 12 more shots down the stretch as the Eastern Conference-leading Pens extended the second-longest winning streak in team history.
Sidney Crosby netted the game's only goal in the middle frame.
Carey Price made 24 saves for the Canadiens, who fell for the third time in four games.