In the second half of Pulp Fiction we find out why Vincent and Jules are dressed in shorts and t-shirts looking like "dorks."
In the start of the movie we witness Jules and Vincent kill Brett and his friend "Flock of Seagulls" for withholding Marsellus Wallace's briefcase. We never really learn what is in the magical briefcase or what deal Brett and Marsellus had, but we do know that Vincent and Jules were ordered to kill Brett and retrieve the briefcase. There was one other individual in the room -- Marvin -- who appears to be a friend of Brett's and answered the door when Vincent and Jules show up.
In the last act we circle back to that same scene to learn that Vincent and Jules were in cahoots with Marvin all along. We also see that Marvin was not the only other person in the room: unbeknownst to the killers, someone else was hiding in the bathroom with a large hand gun waiting to retaliate.
As Vincent and Jules are telling Marvin to keep cool after witnessing the deaths of Brett and Flock of Seagulls, the hidden, would-be assassin jumps out of the bathroom, aims directly at Vincent and Jules and fires six shots. Incredibly, Vincent and Jules find themselves still alive -- the would-be assassin had closed his eyes. They promptly shoot him to death.
Vincent, rather upset, walks directly to Marvin and asks him "Why the *$@# didn't you tell us someone was in the bathroom? Slip your mind? Did you forget that someone was in there with a ... hand cannon?" Before Marvin has a chance to answer, Jules proclaims that what they just witnessed was "a miracle."
As they drive away from the crime scene, Vincent and Jules debate whether what just happened was indeed miraculous. Vincent turns to Marvin to ask his opinion of divine intervention when all of a sudden his gun fires, instantly killing Marvin and splattering blood all over the car.
While the inconsequential mystery of why our anti-heroes later appear dressed so strangely is solved, what is never resolved is a little more profound: why did Vincent shoot Marvin? Was it vengeance for failing to warn them of the assassin in the bathroom? Fodder for more great conversation? Was he trying to prove to Jules that it's just as easy to dodge bullets as it is to get shot square in the face? Could Vincent have really just shot Marvin by mistake?
Tarantino flexes his movie making muscles and gives us resolution only where he wants us to have it. And we are left to ponder the rest.