I feel like what was set up in Empire Vader tempting Luke to join him to overthrow the Emperor was a plot line poorly used in JEDI, or was it dropped completely?
It feels like it was basically dropped completely. From a certain perspective, you can argue that Luke and Vader DID actually team up to defeat the Emperor - but that’s obviously missing the point. When Luke turns himself in on Endor, Vader and Luke have a brief discussion. The subject of overthrowing the Emperor is never brought up for some reason. Vader seems to feel at this point that he has no choice but to turn Luke over to his boss.
Vader’s potential disloyalty to his master is only briefly explored with a single line of dialogue, in the scene where Palpatine tells Vader that Luke will willingly turn himself in. Palpatine asks Vader “Are you sure your feelings on this matter are clear?” (meaning, are you sure you won’t fuck me over and side with your son?) But other than that, it’s never explored. The plot mechanics kind of set things up so that Vader is backed into a corner and has no choice to turn Luke over to the Emperor once Luke turns himself in. It’s really very unsatisfactory in light of what was set up in Empire Strikes Back.