Can we say exactly what happened? for Vader to turn on the emperor?
Not my words....found on tinternet:
"Palpatine was also so confident in how thoroughly he had purged the dark side from Vader, that, along with the fact that Vader was on the dark side for over 20 years now, that there was no chance of him ever turning against him. Of course this shows how ultimately Luke was really more powerful than Palpatine as Luke and consistently sensed the little bit of good in Vader that he was going to try to resurrect. Once Vader saw Luke being killed by Palpatine, and knowing that he could no longer have any children due to his injuries and his condition, finally awoke that little bit of the light side in him that neither Palpatine nor Vader could get rid of. Yes, Vader himself was trying to remove all the remnants of the light side in him, those little bit of compassion, mercy or emotional attachments, but he could never get rid of it all fully. Those are the qualities that Luke kept sensing that Vader was struggling with which eventually reawakened once Luke's like was in jeopardy, and those are the qualities in Vader that Palpatine either failed to sense, or simply didn't think they were strong enough to overcome the dark side in Vader.
Palpatine was simply too confident in the dark side overall as he felt that it would keep Vaders light side from awakening, and if it did, that same power would keep Vader loyal to his Master. As Vader said to Luke "You don't know the POWER of the dark side.. I MUST obey, my Master".
These are the reasons why Palaptine failed to see Vader turning against him at that time, though he knew that Vader was plotting it, for it's the way of the Sith.
Prides goes before a fall."
J