I remember in the RotS book it took the Analogy of the Light side to literal light.
Where there is light, there is a shadow. The shadow is the dark side. the more light, the more shadow. If there is no light, there is only darkness. But it is never like that for long.
I think this explains why Vader could kill those jedi so easily. After they all died, and Yoda and Obi died, there was only Luke (pretty much, but there was Leia.. but I'm not getting into that)
So if the light and shadow analogy is saying what I think it is, There wasn't a whole lot of light and the darkness was 'shared' between Palpatine and Vader. Maybe that's why vader had trouble with Luke?