I agree with a lot of the ideas in this thread. To me, The Emperor is a bit like a mob boss. Tarkin is the head of his "legal" power, working within and through the government structure. Vader is his enforcer who does the nasty work that is clearly outside even the faintest hint of lawfulness. They have a mutual respect for each other because they're two sides of the same coin.

As for Vader's role-shift in Empire, I really like it and think it works very well. He clearly knows who Luke is long before his conversation with the Emperor (to whom he feigns ignorance of that very knowledge). And with this knoweldge, he has gone on a sort of mad, Ahab-like quest to find Luke at all costs.

In his efforts to do so he has used and begun to clearly abuse both the vaccuum left by Tarkin and the power given to him by the Emperor. I don't think Vader's in charge of the ENTIRE Imperial fleet in ESB, but he's used the lenience granted him by the Emperor as well as his own ability to intimidate the navy commanders to take command of small armada for his personal quest and is using the search for the rebels as a sort of cover to justify his real mission.

I think it's also pretty cool how icily cold and in control Vader appears to be on the outside, when inside he's clearly spiraling out of control on a lunatic's mission to find his son & convince him to join and take charge of everything he has been fighting so hard against.