Vader realises, with a great degree of disturbance, that Palpatine has no real interest in turning or training Luke, but is lusting after him instead.

Palpatine had kept this little fact under wraps for a while, but he slips up in Vader's presence, and when Vader realises he decides the Emperor has to go.

Vader takes over the Empire, keeping a reluctant Luke nearby. It's frustrating, because now he barely has the time to train Luke, and he's reluctant to send him to someone else, because so many Siths will only want to use him to gain more power.