Ylonen pushed it to 3-1 at 8:28 when he scored on an unassisted breakaway with a wrist shot past Hill’s glove.
“He’s got really quick hands and a great release,” Canadiens forward Jake Evans said. “So, I like the odds when he’s on a breakaway.”
Brayden McNabb drew Vegas to 3-2 just 18 second later at 8:46 when he shot between Primeau’s legs after Theodore’s point shot hit him and fell in the slot.
Jonathan Marchessault tied it 3-3 at 11:20, taking Ivan Barbashev’s pass on a 2-on-1 and one-timing a shot from the right side.
Ylonen put the Canadiens up 4-3 with his second goal of the period at 11:53.
Theodore drew the Golden Knights even for a second time at 4-4 with a power-play goal at 16:07.
“Their goaltender was excellent, so eventually, if you keep putting pucks at him, you’re going to get something to go usually, and that’s how it played out,” Cassidy said.
Justin Barron scored with 54 seconds remaining for the 6-5 final.
“We can’t think we’re too good to make all the small plays,” Evans said.
NOTES: Canadiens defenseman Arber Xhekaj left in the second after Barbashev’s shoulder check at 13:11 sent him flying into the corner boards. He sustained an upper-body injury and did not return. St. Louis did not have an update after the game. ... It was Theodore's second career four-point game and matched the Golden Knights’ record for most points by a defenseman in a game, which he originally set Dec. 19, 2017. … The teams combined for six goals in the second period, one shy of the most by two teams in a period this season (Carolina at Colorado on Oct. 21: seven in the second). … Barron scored the 12th goal by a Canadiens defenseman this season, tying Ottawa and Carolina for the most in the NHL. … Vegas extended its point streak against Montreal to nine games dating to Dec. 22, 2018 (6-0-3), its longest active run against one team.