pulp fiction - Did Vincent Vega shoot Marvin on purpose? - Movies & TV

In a very popular scene from the movie Pulp Fiction, Jules Winfield (Samuel L. Jackson) and Vincent Vega (John Travolta) are discussing about Jules' will to retire from being an hitman.

They are carrying a guy, named Marvin, in the back of the car, and when Vincent asks him an opinion and turns to talk to him, the gun goes off, killing the guy instantly.

When Jules asks for an explanation, Vincent attributes the "incident" to the car going on a bump in the street. Is there some canon answer about whether this was an intentional shot or not?

By the way, if the answer is "Tarantino left this ambiguous on purpose", I'll consider that an answer too as long as you can back it up with some canon source. By canon I mean interviews where Tarantino states this and/or articles from official sites.

Answer

Searching around, I found an interesting detail. When Jules and Vincent are going to Brett's apartment, they open the trunk of the car in order to get the guns, and this is what they say:

             JULES

We should have shotguns for this 
kind of deal.

            VINCENT
How many up there?

             JULES
Three or four.

            VINCENT

Counting our guy?

             JULES
I'm not sure.

            VINCENT
So there could be five guys up there?

             JULES
It's possible.


            VINCENT
We should have fuckin' shotguns.

Vincent says "Counting our guy?", and indeed, they are talking about Marvin. This explains why they don't kill him in the apartment and they take him with them. He's not an hostage, he's an accomplice. This is not a 100% attack-proof evidence, but if he's their guy, it might be that the shot could be indeed an accident.

Furthermore, on the IMDb page about Pulp Fiction, it is stated that:

According to an interview with Phil LaMarr¹, it was he who came up with the idea of his character Marvin being shot in the face. Marvin was originally supposed to be accidentally shot in the throat and survive. Knowing that Marvin would die a slow, painful death, Vincent and Jules decide that Marvin should be shot in the head and put out of his misery. Knowing that this would make the characters unlikeable, LaMarr took his idea to Quentin Tarantino and he agreed to it, figuring that a single-bullet kill would be funnier.

This kind of proves the accident way, since originally he was meant to be accidentally shot, not in a fatal way, and then be killed by the two hitmen in order to avoid a slow and painful death. But here the death is sudden, yet still accidental. I'll add the interview if I find it. If someone finds it, please post it.

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^{1: The actor who plays Marvin.}