Vanek, the Sabres' leading goal scorer in the regular season, participated in the team's game-day skate Wednesday morning and coach Lindy Ruff was encouraged.
"He's doing well," Ruff said. "I didn't anticipate him skating today. He felt better, so that part was encouraging. We're just trying to get him a little extra ice to see what he looks like."
He was injured in the first period of Game 2 Saturday when he crashed into the boards after getting hooked by Bruins defenseman Johnny Boychuk, and has not played since. He's been sporting a walking cast on his left leg.
Ellis was on the receiving end of an open-ice hit, also by Boychuk, in Game 3 on Monday at the Garden. Ellis was able to return and played sparingly.
McCormick will add a physical presence, which is something the Sabres have been without the past two games. He posted 168 penalty minutes in 66 games for the Pirates this season.
"He's played very well," Ruff said. "He's a guy who is playing 16 to 20 minutes a game down there and he's a big body who can get to the front of the net. He can win battles down low. ... He plays the game real hard."
Joe McDonald covers the Bruins for ESPNBoston.com. Follow him on Twitter.