Vader was worried that Fett would injure the other (more valuable) captives.
According to the film's official [junior novelisation][1], Vader's concern was that in his eagerness to shoot Chewbacca, Fett would hit either Leia or Han, both of whom have intrinsic value.
Realizing what was about to happen, Chewbacca let out a wild howl. He threw his right arm out to his side, striking a stormtrooper with enough force to launch the figure from the elevated platform. Before anyone thought to react, Chewbacca lashed out with his left arm and disposed of a second stormtrooper in the same fashion.
Across the platform, Boba Fett brought up his blaster rifle, but Vader — hoping to preserve his other captives — lashed out and grabbed the rifle’s barrel, forcing the bounty hunter to aim away from the melee.
[Star Wars: The Empire Strikes Back - Junior Novelization)][2]
[1]: http://starwars.wikia.com/wiki/Star_Wars_Episode_V:The_Empire_Strikes_Back(junior_novelization [2]: http://starwars.wikia.com/wiki/Star_Wars_Episode_V:_The_Empire_Strikes_Back_(junior_novelization)