There's nothing stopping them from trading a player off their active roster while in the SCF. But then they wouldn't have the guy they traded, and the guy they acquired couldn't play for them in any playoff games this season, because it's after the trade deadline.
If Vegas wanted Provorov they could tell Briere that they would give him more than the Columbus/LA package once the Finals ended. And if Briere believed that offer was solid, he would have waited and traded Provorov to Vegas.
Silly post.