Vegas took command on home ice with two goals in the first period, followed by two more goals in the second.
"I think we were there from the start, we did a great first period. We might decide the game in the second period where we manage to score two goals. We play good hockey and that's why we win," Karlsson said.
Finn Anton Lundell scored for Florida 14 seconds into the third period. But it didn't help much, as Vegas' Jonathan Marchessault was in front again and shot 5-1 barely two minutes later – his second goal of the game.
Vegas Swede William Karlsson then played forward Michael Amadio who extended the lead to 6-1.
Florida's Matthew Tkachuk reduced the score to 6-2, before Vegas' Brett Howden scored to score two goals.
"It feels good. You want to win every game, so nice feeling, says Karlsson.
With the win, Vegas has a 2-0 advantage in the final series, which is decided in a best-of-seven. The next game will be played in Florida on the night of Friday.
"All games are obviously important. But it's nice to win both at home and go to them with a 2-0 lead, so it's nice," Karlsson said.