Vegas signed Pacioretty to a four-year, $28 million contract (average annual value $7 million) on Monday. He had one season remaining on a six-year contract and could have become an unrestricted free agent July 1, 2019.
The 29-year-old forward had 37 points (17 goals, 20 assists) in 64 games with Montreal last season, missing the final 18 games of the regular season with a knee injury.
The Golden Knights had been targeting Pacioretty since the 2018 NHL Draft in June, general manager George McPhee said.
"Took a couple of months. It's been ongoing," McPhee said. "Sometimes a lot of talk, sometimes quiet. Things picked up in the last 3-4 days. We were getting to a place [Sunday] morning to where a deal was possible, and we had a window of 6-7 hours until about 7 p.m. last night to get a deal done with the player. This deal wasn't going to happen if we didn't have an extension."
[WATCH: Canadiens' video tribute to Pacioretty | RELATED: Fantasy spin | 2018-19 NHL Trade Tracker]
Pacioretty, named the Canadiens' 29th captain on Sept. 18, 2015, said Sept. 6 he would not negotiate a long-term contract with the Canadiens once the regular season started Oct. 3 and was unsure if he would be with Montreal when training camp opened this week.
His agent, Allan Walsh, said Aug. 28 that Pacioretty had no interest in leaving Montreal, but general manager Marc Bergevin said Monday that Pacioretty requested to be traded during the 2017-18 season.
"Last season, there was a request," Bergevin said at the Canadiens' annual golf tournament Monday. "He asked for a trade. I will not go into detail. But that's a fact, yes.
"The reason is, if a player asks for a trade, there's a reason for that. And we accommodate the player. To me, a contract extension was not on the table."
Canadiens owner Geoff Molson would not discuss the motive behind the trade request.
"I don't really want to go into the details of that, but it's been going on for several months now that we've been trying to make this happen, and here we are today, the morning after," he said.
Bergevin acknowledged that the trade was contingent on Pacioretty discussing a contract extension with Vegas.
"That was the condition, and I can tell you that the negotiations intensified in the past 48 hours," he said.
Pacioretty and the Golden Knights play the Canadiens at Bell Centre on Nov. 10.
"It was an ongoing thing all summer, so for it to finally be done, it puts closure to that chapter," Montreal forward Andrew Shaw said. "Both players have a fresh start, so I am sure they are looking forward to that."
Selected by Montreal with the No. 22 pick in the 2007 NHL Draft, Pacioretty has 448 points (226 goals, 222 assists) in 626 regular-season games and 19 points (10 goals, nine assists) in 38 Stanley Cup Playoff games. He has scored at least 30 goals in a season five times, including four straight from 2013-17.
"He can score at an elite rate, and there's not a lot of guys like that in the world," Canadiens forward Brendan Gallagher said.
Montreal (29-40-13) finished 14th in the Eastern Conference last season, 26 points behind the New Jersey Devils for the second wild card into the playoffs. The Canadiens qualified for the playoffs in seven of Pacioretty's 10 seasons, including reaching the Eastern Conference Final in 2010 and 2014.
"You know, it is what it is," Canadiens goalie Carey Price said. "It sounds like both sides are going to be happy. And it's unfortunate to see a player go like Max. I played with him for 10 years. I wish him all the best. I'm sure he's probably excited about going to Las Vegas, and we got a good player in return and we'll see how it turns out."
Video: Golden Knights trade for Max Pacioretty
Tatar had 34 points (20 goals, 14 assists) in 82 games with the Golden Knights and Detroit Red Wings last season. The 27-year-old forward, selected by Detroit in the second round (No. 60) of the 2009 NHL Draft, had two points (one goal, one assist) in eight games in the 2018 playoffs, when Vegas lost to the Washington Capitals in five games in the Stanley Cup Final.
"His statistics for the past four or five seasons are around 20 goals per year, so he's a player who brings an offensive aspect," Bergevin said. "And with Max's departure we felt we needed to replace that, which we did, and we also got a second-round pick, so now we have 10 picks at this point for 2019."
Tatar was acquired by Vegas in a trade from Detroit on Feb. 26 for a first-round pick in the 2018 NHL Draft, a second-round pick in the 2019 draft and a third-round pick in the 2021 NHL Draft. He has three years remaining on a four-year contract he signed with Detroit after the 2016-17 season.
Suzuki, selected by Vegas with the No. 13 pick in the 2017 NHL Draft, had 100 points (42 goals, 58 assists) in 64 games for Owen Sound in the Ontario Hockey League last season. He signed a three-year, entry-level contract with the Golden Knights on July 15, 2017.
"One point that was very important for us was to get a young prospect, what we call an 'A' prospect," Bergevin said. "So that was a key that was very important to the organization and that's what we were able to get in Suzuki."
NHL.com staff writer Mike Ziesberger and correspondent Sean Farrell contributed to this report.