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.