Ok, so the general feeling about this is that Vincent was in a bad mood and was just having a pop for no real reason. Some way or another he was aware of Butch's deal with MW and decided to mouth off about it to release some of his anger and frustration.
But I believe there is an important reason for QT adding this little exchange in, mightily important in fact.
Butch has taken his cash off MW and is prepared to throw the fight. Throughout the whole chat with MW he shows no emotion, just a cool, laid back attitude. He has nothing against MW and just wants to do his job and collect his cash.
However, after the slur from Vincent, look at his reaction again. His face drops, he is shaken, his pride hurt to the core. He then watches intently as Vincent is welcomed by MW with a big double hug. Vincent is obviously MW's main man. Butch continues to throw glances and is obviously still hurt and deep in thought. There is no doubt in my mind that in that moment he makes the decision not to throw the fight. He has now associated all his hurt and anger from Vincent's insult with MW. Maybe he's thinking about all that talk of pride that MW has just spouted, how it means nothing, how he should ignore it, yet all the while he has probably been laughing about him with Vincent.
Now he's angry, angry with both of them, hurt and his pride already dented before the fight has even begun. No, now he will not throw the fight. He'll prove Vincent wrong and make fools out of both him and MW.
This 10 second piece of the movie is possibly the most important of all, as it sets in motion a dramatic sequence of events that leads to life changing consequences for all three characters.