The Flyers are making things difficult on themselves as they try to stay in the race for the Metropolitan Division title.
They played solidly Monday night but lost for the sixth time in seven games, dropping a 3-2 decision to upstart Vegas at the Wells Fargo Center.
Ryan Carpenter scored from the left circle to snap a 2-2 tie after Cody Eakin won a board battle behind the net and threaded a pass to the third-line right winger with 2 minutes, 40 seconds left in regulation.
"We've go to do a better job on the battles," right winger Wayne Simmonds said.
Radko Gudas, Andrew MacDonald, and Val Filppula were the Flyers behind the goal line.
Marc-Andre Fleury made 38 saves and became the 12th goalie in NHL history to register his 400th victory.
It was the second time in three games that the Flyers lost late in the third period. They allowed a goal with 22 seconds left in a 3-2 defeat in Boston on Thursday.
The lost points could be killers.
"We're letting these games get away from us. It's March, we can't be doing this," Shayne Gostisbehere said. "We need every point we can get."
"We have to refocus," Simmonds said.
The Flyers had a 40-29 shots advantage Monday, but you don't get points for shots.
"We played solid, but we didn't get the two points and that's what matters, right?" said Simmonds, whose team fell four points behind Metro-leading Washington.
Vegas was 2 for 2 on its power play, while the Flyers were 1 for 4.
"We played a good game, five-on-five," center Sean Couturier said. "Our [penalty kill] needs to be better. We have to move on and forget about this one as soon as possible."
The Flyers are six points ahead of Florida, which has three games in hand and is fighting to get into the last wild-card spot.
Simmonds tied the game at 2-2 by scoring from close range with 12:54 left in regulation, beating Fleury with a perfectly placed shot to the short side that went over the goaltender's right shoulder while the Flyers were on a power play.
The Flyers were in a 2-for-28 power-play funk before Simmonds converted a slick feed from Jake Voracek.
William Karlsson had given Vegas a 2-1 lead by scoring on a power-play rebound with 14 minutes left in the third period. The Golden Knights capitalized on a high-sticking penalty against defenseman Travis Sanheim.
Ten seconds before his penalty, Sanheim was robbed by Fleury, who gloved his ticketed shot from the high slot.
Claude Giroux's 10th goal in the last 16 games enabled the Flyers to tie the game at 1-all with 18:33 left in the second period.
Travis Konecny took the puck away from Shea Theodore near the offensive blue line, and Sean Couturier fed Giroux for a left-circle wrist shot that sailed over Fleury's glove for his 25th goal, three shy of equaling his career high.
About two minutes later, former Flyer Pierre-Edouard Bellemare leveled Nolan Patrick, and the rookie had to go to the locker room and be tested for a concussion. Patrick returned to the ice about five minutes later.
The Flyers were the stronger five-on-five team – holding a 9-4 shots advantage — but Vegas won the special-teams battle in the opening period.
Vegas had five shots on its power play, including a pass by Erik Haula that deflected off the stick of goaltender Petr Mrazek and into the net. Haula was at the goal line to the right of the net when he tried to throw a pass out front that Mrazek misplayed.
The Golden Knights, an expansion team that leads the Pacific Division and is setting many NHL records, got the power play when the Flyers were whistled for too many men on the ice, a call that infuriated Dave Hakstol, their normally placid coach.
Fleury made a key save on Couturier (redirection) in the first period, and made a glove save to rob Voracek from scoring on a left-circle drive with 8:38 left in the second. With a little over three minutes remaining in the second, he made consecutive point-blank saves on Patrick and the snake-bitten Oskar Lindblom (rebound).
Mrazek, Fleury's counterpart, denied Alex Tuch as he broke in alone early in the first period, and he stopped Haula after Vegas had a three-on-one following a MacDonald turnover with 15:26 to go in the second.
The Flyers controlled the second period, scoring the session's only goal and having a 16-8 shots domination against a team that was completing a five-game road trip.