The winning goal came moments after the Blues thought they had taken the lead.
Brad Boyes put the puck in with 16:46 left and moments before play was stopped when Senators goalie Martin Gerber took possession of the puck. Replays showed that the puck went about a foot past the goal-line before Gerber pulled it back with his stick and smothered it. The goal light wasn't turned on and the officials didn't look at replays until after play resumed.
According to Rule 78.6, once a game has restarted following a stoppage of play - in this case when Gerber held onto the puck - a replay can't award or disallow a goal.
The Blues lost another apparent goal with 2:18 left when officials said David Backes' shot was covered by players. Again, replays showed the puck went over the line and was pulled out by Gerber before the whistle blew.
Vermette's 19th of the season was the second short-handed goal of the game and it came just nine seconds after Mike Fisher was sent off for delay of game. Vermette took a pass deep in the Blues zone from Chris Kelly, and goalie Curtis Sanford failed to slide to his left fast enough to stop the shot.
Mike Fisher got the first short-handed score midway through the second period. Fisher settled the puck at mid-ice before fighting off Lee Stempniak as they entered the Blues zone. He slapped in his 19th goal to give the Senators a 2-1 lead.
Kelly tied it 1-1 with a his 15th goal 6:29 into the second and added an empty-net goal to seal the win.
The Blues, who have failed to win during regulation in their past five games, opened the scoring on Barret Jackman's second goal of the season.
Boyes tied the score 2-2 with his 16th goal late in the second.
Notes: Blues C Ryan Johnson played after sitting five games because of an upper body injury. . . . With an assist on Boyes' goal, Doug Weight has 20 points in 19 games against Ottawa. . . . This was the only meeting between the teams this season and the first since Feb. 10, 2004, in Ottawa. The clubs last faced off in St. Louis on Dec. 5, 2002.