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.
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?