We know how this played out; Luke and Vader fought, Luke dealt Vader a mortal blow (or did the force lightning finish him off?), Palpatine attempted to kill Luke, and Vader finally made up his mind about where his loyalties lay and saved Luke.
But what if, when Vader stopped Luke's strike, it was not to save his master, Palpatine, but to stop Luke from taking that final step to the Dark Side?
As is commonly considered, the Emperor goaded Luke into raising his lightsaber against him, and in doing so this action caused Vader to intervene with his own lightsaber and save the Emperor's life.
He's started to wear black. He's slowly becoming part machine due to his dark daddy cutting off his arm, and earlier in the film, he Force chokes a couple of Gammorean Guards at Jabba's Palace. So he's ripe for the turning right?