The Emperor tells Vader that only together they can only turn Luke to the dark side. Luke willing to die instead of joining his father in Empire proved to Vader that he can’t do it without the Emperor. I think the Emperor’s hold on Vader did play a part as to why Vader would defend him, but I think Vader did wanted to kill the Emperor, but only after his son joined him on the dark side. At the point Luke grabbed his saber and tried to kill the Emperor, I don’t think Vader believed that would’ve been enough to turn Luke. Luke needed to be angrier, give in more to the dark side, before he would succumb. But I think Vader didn’t see it would end with Luke almost killing him instead. I think Luke snapping when he threatened to turn Leia caught him off guard.