Tkachuk extended the lead to 5-0 at 3:09 of the third period when he pulled the puck on the goal line between his legs before lifting a shot over Lehner. He has scored 26 points (10 goals, 16 assists) in his past 16 games.
"We knew they were waiting for us," Vegas forward Jonathan Marchessault said. "They've been waiting for that game for a while and we came off just flat. There's no excuse for our group. We're a veteran group and we know how to handle those situations, and we definitely didn't do it tonight."
Mangiapane made it 6-0 with his team-leading 22nd goal on another setup by Backlund on the power play at 15:44.
"That was a big win for us," Backlund said. "It's important for us to have a good start and we got what we wanted. A little sloppy second period, 'Marky' was playing real well for us in the second and held us up 4-0, so that was huge for us going into the third, and I thought we had better jump in the third."
NOTES:Miikka Kiprusoff holds the Flames record for shutouts in a season with 10, set in 74 games in 2005-06. ... The Flames are the fastest team to 10 shutouts in a season since the New Jersey Devils in 2003-04, according to Sportsnet. ... Mangiapane has scored 19 of his 22 goals this season on the road. ... Golden Knights forward Mark Stone did not play because of an upper-body injury. He will be evaluated in Vegas, according to DeBoer. ... Vegas forward Brett Howden had a personal four-game goal streak end. … Vegas was 0-for-2 on the power play and is 1-for-18 (5.6 percent) in the past seven games.