Like 0-0-0 and BT, Boba Fett also worked with Vader, in the EU extensively. Vader is also shown to be quite liberal in his use of Bounty Hunters in both EU and nucanon. It's just part of what he does, (and other imperials often disapprove of this practice, hence the restriction).
Cooperations less significant than this have been represented in the mechanics several times. Remember, Boba Fett found Solo as an imperial agent (in the sense that he was employed by them at the moment).
This way, with a strong limitation (having Vader in your list is pretty much half of your points, or a crew carrier with -1 crew slot) the Empire's array of crew and gunner upgrades could be extended, while also maintaining flavor.
I could also see something like this:
Corrupt Imperial Officer
You may equip Scum only crew and gunner upgrades
You gain the [illicit] slot
Obviously, if such themes are extended, I'd be happy to see it with other factions as well. Rebel Kallus, etc.
