William Karlsson had a goal and an assist in his 600th NHL game, and Alex Pietrangelo also had a goal and an assist for the Golden Knights (41-20-6), who have won three straight and were playing the second of a back to back following a 4-0 win against the Carolina Hurricanes on Saturday.
Vegas moved three points ahead of the Los Angeles Kings for first in the Pacific Division.
"For playing four in six days, the mental fatigue, the travel, we did a good job," Golden Knights coach Bruce Cassidy said.
Jordan Kyrou had a goal and an assist, and Jordan Binnington made 26 saves for the Blues (29-32-5), who were also playing teh second of a back to back following a 5-2 win against the Columbus Blue Jackets on Saturday.
"Obviously, the energy was there, the passion, the fight was there," St. Louis defenseman Torey Krug said. "We came up short."
Karlsson put the Golden Knights ahead 1-0 at 8:05 of the first period, getting to a loose puck in the crease and finishing with a backhand past Binnington's outstretched glove.
The play was created by Dorofeyev, who forechecked St. Louis defenseman Nick Leddy into a turnover.
"He was on the puck a lot," Cassidy said of Dorofeyev. "He created a turnover on the first goal with foot speed. I thought on his off wing he was able to get in on the inside a lot, to be dangerous there, went into traffic. He was kind of slippery around there."