Vanek is still nursing a lower-body injury suffered in Game 2. He has not played since.
The Bruins lead the best-of-seven series 3-1.
Vanek, Buffalo's top goal scorer, was injured Saturday after he was hooked and slashed by Boston defenseman Johnny Boychuk in the opening period of Game 2. Vanek slid hard into the end boards and since has had difficulty putting weight on the injured foot.
"If he felt he could play and be effective, he'd be in the lineup tonight," Sabres coach Lindy Ruff said. "We have one game to stay alive. In the conversation with him, he didn't think he was at that point yet."
Ruff will shake up his lineup for the pivotal fifth game, inserting Nathan Gerbe for veteran winger Raffi Torres. The diminutive Gerbe (5-foot-6, 160 pounds) played in 10 games this season for Buffalo, scoring twice and registering five points.
"Hoping he can bring some scoring," Ruff said of the former Boston College star. "Maybe a second unit power-play guy who can add a little something for us.
Torres, who will be a healthy scratch, was acquired by the Sabres at the trade deadline in March, and didn't score a goal in 14 games. In the playoffs, he has two assists, but no goals.
Information from The Associated Press was used in this report.