In Star Wars I think of Tarkin as the main villain, with Vader as a subordinate. I thought it worked perfectly and made for a much better story.
Lucas started to morph the character as soon as his pop culture weight grew. In The Empire Strikes Back, he's just suddenly a person who answers to no one other than the emperor, unlike Star Wars where he was part of the Empire and participated in strategies and discussions. In the second film he's just someone who people are terrified of.
He runs the Empire in it's entirety other than an occasional conversation with the emperor. It's like the Empire was restructured between films. It became a two-person operation with thousands of minions, not the stratocracy it was in Star Wars.
His flippant killing of anyone he dislikes and his full authority to promote anyone to commander seems uneven with how he was written in Star Wars. By the second film, there's no depth to the Empire. To me, it comes across as more of Lucas' letting marketing and revenue stream drive the films.
[shameless EU plug]
Yet another way in which the EU has far surpassed Lucas Star Wars. In Zahn's EU, the Empire is multi-layered, much the same way an actual military is. There are personalities and hierarchies to deal with. Levels of authority that have to be coordinated. It makes for much more interesting reading. Daley's NPR version of Star Wars also has that layered depth of the Empire with Vader as part of it, not the sole voice of it. With Lucas (post-77), the Empire became one-dimensional. Vader sold tons of toys, so Vader became The Empire.
[/shameless EU plug]