Rewatching Pulp Fiction, it's really hitting me how terrible Vincent is at his job. His incompetence causes nearly every hiccup in the entire movie.
* Just spent the last three years in Amsterdam, possibly laying low based on a previous screw up.
* Fails to check the back room of the apartment that he and Jules raid for an armed man, who nearly kills him if not for the "miracle."
* Accidentally shoots Marvin in the face. Any competent gun owner would avoid this (hand on the trigger, pointing it without intent to shoot).
* Disrespects The Wolf, the only man with the know-how to fix the "Bonnie Situation," a situation that he himself caused.
* Has a substance abuse problem, and does work for his boss under the influence of said substance, nearly killing Mia.
* Leaves gun on the counter of Butch's kitchen as he uses the bathroom. He was reading in the bathroom, so he was most likely there for a while. Of course, this leads to his demise.
So Vincent isn't exactly the best hitman. So why does Marcellus trust him so much? Well, think about it, he really doesn't give Vincent that much responsibility through the film.
* Job 1: Go with Jules (a very competent hitman, whose relationship with Marcellus is so good that he can yell at him when things get heated during the Bonnie Situation) to kill three men who are keeping the "briefcase' from me. During this job, Jules basically does all of the work.
* Job 2: Take my wife to dinner while I'm out of town. Literally, just drive her to dinner and take her home safe, basically babysitting.
* Job 3: Go wait in Butch's apartment just in case he shows up. I mean, his stuff is all packed up and he probably isn't going to show up again, so... just hang out?
Remember, Vincent has only been back in America for a few days at this point. These are the only three jobs that he's been given and, honestly, they're all pretty simple for that line of work.