Michael Amadio, Alex Iafallo and Jeff Carter scored for the Kings (14-20-3), who were coming off a 3-2 overtime win at the San Jose Sharks on Saturday. Cal Petersen made 22 saves.
Los Angeles has won the first two games of the season against Vegas after being swept by the Golden Knights in the Western Conference First Round last season.
"Anytime you're playing a team in your division (Pacific), your goal is to beat them," Toffoli said. "We've played well both times against them. That was a really big game for us going into the break, and it's a really good feeling right now."
Tomas Nosek, Reilly Smith and Ryan Reaves scored, and Fleury made 28 saves for the Golden Knights (20-15-4), who lost 4-3 in overtime to the Montreal Canadiens on Saturday. Vegas is on eight-game point streak at home (6-0-2).
Fleury started after making 43 saves on 47 shots against Montreal.
"I felt decent, actually. It's a grind sometimes," Fleury said. "We've done it throughout this year, and it's no reason for losing the game."
Nosek scored at 10:10 of the first period to give the Golden Knights a 1-0 lead.
Amadio tied it 1-1 at 3:32 of the second period, but Smith scored at 10:01 to give Vegas a 2-1 lead.
Iafallo scored 2:51 later to tie it 2-2 at 12:52.
Carter gave the Kings a 3-2 lead with a power-play goal at 2:06 of the third period, but Reaves tied it 3-3 when he scored on the power play at 6:55.
The Kings go into the Christmas break last in the Western Conference, eight points behind the Edmonton Oilers for the second wild card into the Stanley Cup Playoffs.
"The division games are huge," Toffoli said. "Huge points right now. [We're] not as far away as everyone thinks."
