In Pulp Fiction (1994), when Butch goes back to get his watch, he enters the apartment. After perceiving that he is alone, he relaxes, starts moving normally, even makes a pop-tart.
Wouldn't all of this movement have caused enough noise for Vincent to hear? When the door opens, Vincent has the look of a man unaware of his surroundings. I would expect that not only he would have heard Butch, but try to be more discreet about getting out of the bathroom.