Vanek is still nursing a lower-body injury sustained in Game 2.
Sabres coach Lindy Ruff will shake up his lineup for the pivotal fifth game, inserting Nathan Gerbe for veteran winger Raffi Torres. Gerbe played in 10 games this season for Buffalo, scoring twice and registering five points.
Torres was acquired by the Sabres at the trade deadline in March, and didn't score a goal in 14 games.
The Bruins lead the best-of-seven series 3-1.