Vader stopped Fett from firing because Vader was still in control of the situation, and he didn't need Fett to shoot Chewbacca. He was continuing to dominate the situation and get what he wanted, the way he always did. Vader's intention was to get Luke to join him, and part of his plan was to lure Luke using his friends and making them suffer, and not to kill off Luke's friends. Chewbacca wasn't going to get away, or overpower Vader, and the armored stormtroopers he tossed were probably just bruised and aren't important - he had plenty of them around.
Yes, Luke was already on the way, but that didn't mean their suffering wasn't still useful as bait.
And, since Vader wanted to convince Luke to join him, not killing his friends might be helpful.