Why doesn't Vincent freak out after he accidentally shoots Marvin?
Not that he didn't at all freak out, but he's considerably calm about what just happened when compared to Jules and his reaction.
I don't know if that was the intent, but I feel horrible for finding the scene darkly funny. Maybe a lot of it has to do with how casual John Travolta makes it sound like ("I just shot Marvin in the face!"). He makes it sound like a minor nuisance like accidentally spilling a cup of coffee and staining your carpet.
Maybe, since Vincent is a professional hitman (i.e. he kills people for a living), he sort of become desensitized to the seeing blood and murder. And I would assume that hitman by nature, have to be somewhat emotionally detached in order to function, which could be why Vincent seems so composed.
If anything though, Vincent is more panicked when Mia ODs while on his watch than him accidentally shooting a man to death.