Character development is more than whether one is evil or not. (or looking 'stoopid')

Its contamination from the prequels thats clouded your judgement!

No it's not! AND YOU'RE BEGINNING TO SOUND LIKE A SEPARATIST!

RE: Darth Vader- not enough screen time to be major character

Luke, Han, Leia and even Yoda, Chewbacca and R2-D2 (probably even Lobot) get more screen time than Vader, so it's true- and I think I already said this... going by the standard definition, he's clearly not the main character of the film. However, I think if you look at who is driving the plot, and whose problem is "the problem" of the film, I think you have to conclude it is Vader. Vader is not the protagonist of the film, but he is the protagonist of the problem which is at the center of the 3 acts upon which the film is hung.

Which should come as no surprise, really. It's very common for the Antagonist to rule over Act 2. So again returning to the Macro Acts for the trilogy, we see that Vader rules over all of ESB, and that he is the Protagonist of the problem, even if we don't see the story from his perspective.

It can be a little like arguing that since Sherlock Holmes stories are told from the perspective of John Watson, and that he indeed is in more of the book than Holmes... that Watson has somehow become the main character of the book.

Watson is the perspective character. Holmes is the main character.