Marchessault made it 4-1 at 10:56 of the third when he was awarded a penalty shot after getting slashed by Manson.
Silfverberg scored at 14:34 of the third to make it 4-2.
"They were the better team for 60 minutes," Silfverberg said. "Getting that one towards the end of the third, you get a little bit of life. You never know what's going to happen. [Gibson] was terrific tonight, but the rest of the team didn't play as good. It was a tough game."
Marchessault scored an empty-net goal with 1:54 remaining for the 5-2 final.
"I think the last six weeks, there might have been a game or two we didn't like, but for the most part, we've been real good," Golden Knights coach Gerard Gallant said. "I don't think we've had a bad season. I'd like our record to be a little better than what it is, but we've been playing good hockey."