Juuse Saros made 45 saves for Nashville (34-22-5), which lost for the fourth time in five games and trails the Winnipeg Jets by three points for first in the Central Division.
"I can tell you, it wasn't a good game," Predators coach Peter Laviolette said. "We were outdone in a lot of areas, and the score reflected that."
Nashville's Colton Sissons scored at 15:47 of the first period, but Vegas coach Gerard Gallant challenged for offside, and the goal was overturned after video review showed Sissons entered the zone ahead of the puck.
The Golden Knights took a 1-0 lead at 17:17 of the first when Cody Eakin's lob pass from center ice went to Saros, who couldn't control the puck, and Pacioretty scored on the rebound.
Brandon Pirri scored a power-play goal at 10:53 of the second period to make it 2-0.