It was always my interpretation, not that any of this came from anything official I don't think, that Vader, as "Dark Lord of the Sith" with a "sad devotion to that ancient religion" still had some code of ethics that he lived by. Can't say exactly what that was... But the Emperor sort of just used/abused whatever power he could. That he had no formal Jedi training, and that he had no specific Sith religion either. That he was a "wilder", to borrow a phrase from the Wheel of Time. He had Force sensitivity and he taught himself or learned from others without following them all of the tricks he needed to get along. And maybe that the Emperor in that way kooked Vader out a little. I'm thinking of the Sheriff of Nottingham and the Witch in Robin Hood Prince of Thieves. He's using her to his own means, but he actually finds her and her witchcraft somewhat repulsive due to this mostly traditional mindset.

So I was a little put off when the PT showed him as the head of the Sith order, with his own "Darth" title as well.

And speaking of things that would have been brought up in the OT had they actually existed in the PT... "Darth Sidious", anyone?