Stone tied it 2-2 on the power play at 12:52. Max Pacioretty had an assist for his 600th NHL point, becoming the 10th active United States-born player to reach that mark.
Alex Tuch scored at 19:10 to give the Golden Knights a 3-2 lead.
Marchessault scored his second at 13:18 of the third to make it 4-2, and Mattias Janmark scored into an empty net at 19:04 for the 5-2 final.
Vegas has made the playoffs in every season since entering the NHL in 2017, including an appearance in the Stanley Cup Final during its inaugural season.
"The first year, we were just concentrating on winning four games, probably," Marchessault said. "I think it's something our group and organization should be proud of. Four straight years in the playoffs, that's how you build a franchise."
NOTES:Vegas is the fourth team in NHL history to qualify for the playoffs in each of its first four seasons (Edmonton Oilers, 13, 1980-92; New York Rangers, 9, 1927-35; St. Louis Blues, 6, 1968-73). … Marchessault also scored four points (two goals, two assists) in a 7-3 win against the Sharks on March 18, 2019. … Pacioretty has 10 points (three goals, seven assists) in a six-game point streak. He has 302 goals and 298 assists in 807 NHL games in 13 seasons with the Golden Knights and Montreal Canadiens. … Fleury won his 487th NHL game. He is two wins behind Roberto Luongo for third in NHL history. … Vegas also won eight in a row from Dec. 14, 2017-Jan. 2, 2018. … Stone had his five-game multipoint streak end. … Janmark scored his first goal in his fifth game with the Golden Knights after being acquired in a trade with the Chicago Blackhawks on April 12. … Korenar is 0-2-0 in four appearances this season. The 35 saves were an NHL career high for the rookie.