Boba Fett is a bounty hunter, he works for credits.

He would know Vader/Empire wants Luke based on the events of Cloud City. So it would be financially practical for him to go straight to the empire/vader with that info. He has credit with Vader, so there could be an “open” communication, or at least Vader would either take Fett’s calls or take the information seriously.

But that is also a good point Vader would not leave the Death Star II to search for Luke based on the Emperor’s orders.

Hmmm this one is a thinker.