"Two there should be; no more, no less. One to embody power, the other to crave it." ―Darth Bane
Did Darth Vader REALLY crave power? After thinking about the films a bit I realized that Darth Vader didn't really explicitly act in a way to obtain power for himself. The more I think about it, it seems like he got snared in a situation in which he realized that he was on the wrong side but didn't see a way out.
It seems like he keeps doing bad things not out of a desire for power, but out of some adolescent desire to please an authority figure/parental figure that is perpetuated throughout his adult life. He expresses regrets to Luke about the way things are.
I think the most telling point I have to make is that when Vader is fighting Luke on Death Star II he uses his lightsaber to keep Luke from killing the Emperor. Wouldn't a REAL Sith have killed the Emperor and used the ensuing confusion to capture Luke, to brainwash him into being his apprentice?