Anakin always planned on using the Emperor for his own ends and then discarding him, just as the Emperor plans on using his apprentices for his own ends and then discards them.
With Luke entering the picture, Vader had "A New Hope" of being able to finally take Palpatine out of the picture, which is why he gives a similar pitch to Luke (as he had given Padme) on the gantry in ESB. Between ROTS and ESB, Anakin was a shell of his former self, and didn't have the power to destroy the Emperor on his own.
Palpatine also sees Luke as the apprentice he had hoped Vader would have become before he lost all his limbs and needed a respirator to survive.
"How is that possible?" = "Hey I wasn't looking for him already to help take you out, honest!"
"Yesss. he would be a great asset" = "As soon as I convert Luke, you're ass is grass Vader"