So why didn't the FBI agent arrest Vin Diesel?

[Thrifty]

Furthermore, didn't his superiors at the FBI know that he was consorting with this wanted fugitive?

The FBI didn't need Diesel's help.  He didn't bring anything to the table.  He was just as much an outsider to Braga's gang as anyone in the FBI.

[Quicksilver]

did you watch the first movie? That would explain their relationship.  They needed diesel because of odd circumstances really. First run in diesel gets away because O'conner was saving the other criminal, second time he couldn't blow his cover in the club. Third time O'Conner thought he might need diesel till he just busted that one guy to get a spot in the race. Lastly O'Conner didn't arrest him after the drug run because of their past and O'Connors guilt for entangling diesel's sister in the original drug sting. again watch the first movie. If you haven't seen the 1st than lets just say they're kind of friends

I can't believe I just explained something like that. If when I woke up someone said guess what movie's plot/character relationships were this would this would be my millionth guess  I actually only like this movie for the hot NSX in the last scene and to see Clint Eastwood's Gran Torino, btw that was the only car to survive the entire movie. Now to watch the rifftrax version. I'm excited 

Now why the douche bag FBI agent didn't just arrest him Idk, guess he would have gotten his ass kicked?

[Plastic Self-Cleaning Duck]

Same reason Utah didn't arrest Bodhi in Point Break, I guess.

In fact, that guy who does "Bad Movie Beatdown" points out these two are just the same movie......

(PS - didn't spoiler about Point Break since nothing else is being spoilered)