Yes, but Boba had much better aim than your average stormtrooper (a five-year-old with a squirt-gun has better aim than your average stormtrooper). He would've been able to hit Chewie without damaging the droid.
I think Vader stopped him because he (A) knew that the wookiee was, at that point, controllable, and (B) wasn't confident that one blaster shot would be enough to take down a really p*ssed off wookiee.
"I'll shoot him."
"On stun! I'd rather have an angry wookiee than-- <small>What am I saying?</small> Full-power!"
--from an RPG
It's like I always say: When the going gets tough, the tough . . . switch to artillery.