The MAC-10 isn't Vincent's, it belongs to Marsellus, who is staking out Butch's apartment with Vincent, but has gone for coffee and donuts, breakfast for himself and Vincent-Marsellus wouldn't have been able to conceal the MAC-10 on his person very easily if he'd taken it with him to get breakfast. Butch is extremely fortunate: his timing couldn't have been better when he entered his apartment while Marsellus was gone and Vincent was in the bathroom. Vincent probably hears Butch come in, but believing it is Marsellus, is not alarmed. As Butch is driving away after having retrieved his watch and killed Vincent, he encounters Marsellus on his way back to the apartment, carrying a box of donuts and two cups of coffee. The "trivia track" on the DVD confirms this interpretation to be correct. An added explanation that does not disagree with the above but adds a psychological dimension is that Vincent is demonstrably very careless with guns. In another scene, this carelessness costs someone else (Marvin) his life. Here, Vincent carelessly assumes that Butch would not be dumb enough to come back to his own apartment and, so, allows the gun to go unattended in the kitchen while he uses the facilities. Also, Marsellus and Vincent could have been waiting for anyone to come to Butch's apartment to interrogate them as to Butch's whereabouts: Butch could have sent Fabienne to get his watch thinking Marsellus and Vincent wouldn't know who she was. (Also, Vincent is seen to be using the facilities more often than any other character in the movie, at least three times, twice while reading the same book. The theme throughout the film is that whenever Vincent goes into the bathroom, something bad happens: Mia overdoses on his heroin, he's killed by Butch, and he finds himself in the middle of Pumpkin & Honeybunny's robbery of the restaurant.) It's also not out of the realm of possibility that Vincent didn't even know Marsellus had put the MAC-10 on the counter. If Vincent likes to read while using the restroom, it's entirely possible he was in there a while and may have been in there before Marsellus even left. Heroin addiction is known to cause constipation making that even more likely. Edit