That changes everything. Marsellus might be willing to forgive Butch for ripping him off, but there's a good chance he'll change his tune once he realizes Vincent is dead. Remember, this wasn't just any random henchman -- Marsellus trusted Vincent so much, he asked him to go get hammered with his hot wife. We wouldn't trust our own ED-suffering grandpas with that.
Hell, even if Marsellus doesn't want to kill Butch, he still kind of has to. Once word gets around that the guy who killed a beloved member of the Wallace criminal organization is out there, acting like he has nothing to worry about (because he honestly thinks he has nothing to worry about), Marsellus is gonna look like a chump. One day, one of those henchmen Marsellus apparently pays to take years-long vacations is going to see Butch at a supermarket in Colombia or something, and he'll have no choice but to order a hit.
"Oh, and find out what they call the Whoppers over there before you come back."