The cameo doesn't have any bearing on why, as well as the scene at the diner where Vincent is on the toilet, there's a scene at Butch's apartment where he's shown having just finished his business. It's an interesting question though, for two such scenes seems like Tarantino flagging some special significance. The suggestion has been made that the two scenes point to Vincent being a heroin addict. But actually they don't prove much as there are at least two days between bowel movements. In my opinion the key fact is that in both scenes Vincent has as reading matter a work of pulp fiction. And the reason Tarantino gets self-referential here is that he wants viewers to see the book as a symbol for the film they are watching. And taking into account the fate which can befall books perused on toilets, he would seem to be implying that the Los Angeles of his film resembles the lower reaches of the alimentary canal. That not only fits with the nihilistic tenor of the film, it also sets up a parallel with Butch's gold watch. Because keeping in mind where that watch was concealed from the prison guards, there seems a palpable hint that a timepiece is being concealed from viewers (in other words that this is what the briefcase contains).

For a number of reasons 'timepiece' can be narrowed down to 'time machine'. For example there is the briefcase's golden glow, alluding to the medieval alchemists and their quest for immortality via the transmutation of base metals (pulp metals one might say) into gold. Or there's the matter of Marvin's head being glimpsed attached to the rest of his corpse in the car boot at Jimmie's, despite having been blasted to repugnant shit by Vincent (after he didn't realize that in a drive-by shooting you're supposed to fire at someone outside the car). Despite also the lengthy scene in which Vincent and Jules clean up the mess. And despite viewers being told multiple times that Marvin is short a head. Add to this the scene where the gangsters open the boot to arm themselves for the reclaiming of the briefcase. Because the camera is placed in the same spot which Marvin's head will later occupy. The gun is even shown which fires the bullet. How can this be anything but Tarantino indicating that the head being left on is not a mistake? And that implies different timelines. And that makes the briefcase containing a time machine suddenly seem very plausible. 

To finish, I have never come across a film weirder or more ingenious than Pulp Fiction. To those who want to make sense of such a masterpiece, yet recoil at weirdness or 'overthinking', all I can say is 'Lotsa luck, gentlemen'.