In Pulp Fiction, Jules starts laughing at Vincent’s answer when asked if he met Mia. Why does he start laughing? I never really understood the joke or what made it so funny.
As A.bakker explained, Marsellus is known to be jealous. But the main issue is that Vincent thinks it will be a straightforward assignment, and he would have no problem avoiding anything that would make Marsellus jealous.
BUUUUUT he hasn't met Mia, who is, well, not exactly boring, is she? His plan to
sit across a table, chew [his] food with [his] mouth closed, laugh at her jokes and that's all [he's] gonna do is not as easy as he thinks.
He's laughing because Mia is Marsellus' wife, and he is known to be jealous.
In a previous conversation between Vincent and Jules (While driving to get the briefcase) they discussed his jealousy, allegedly throwing a man off a balcony for giving his wife a foot massage. Vincent dismisses it as an urban legend/exaggeration stating that he isn't nervous and nothing special will happen. While Jules implies that him taking out Mia might be a suicide mission due to the jealous nature of Marsellus.