As Darth Vader and the Emperor transition the Republic into the Empire.. Vader is not living up to his "chosen one" status..
I believe the Emperor is looking for new talent and diversifying his portfolio with new apprentices or generals, intelligence, mercenaries...to make his Empire for effective and effecient...

Darth Vader is networking for himself for future plans to overthrow the Emperor..
That is where Boba Fett comes in.. Boba Fett is the perfect weapon for Vader with his intelligence, efficience, and vendetta against Jedi.. Vader can take care of "off the record" Sith/Empire business without getting his hands dirty..
The Vader's comments to Fett in ESB suggest a prior history of working together toward a common goal through different means.. "No disentegrations"...
Fett gets the job done... delivering the Falcon to Vader on Cloud City and dropping off the Solo goods to Jabba..

Comments?
Thanks,
CE