I know for some this scene is offensive but, the comedy of this scene is not really about his stutter. It is about how the horror on Stan's face. He thinks Vinny is a joke and this lawyer is his only chance to win. When he can't even get through questioning the first witness he realizes the jury may see the lawyer as a joke. Especially, when his line of questioning does nothing but, strengthen the prosecution's case. Follow this up with Vinny decimating the witness and proving his testimony is not accurate and you have an awesome scene. Then you add in the fact that the actor deals with this problem in real life and chose to show this does not mean you should give up on your dreams.