One problem I've always had about Darth Vader in the OT

I think by this point he is more content with his situation, and more "in control" than when he was a "rage monster" after first turning to the Dark Side. He's not going to put forth more effort than he needs to, most likely.

Interesting, perhaps it was more despair that was feeding him in his alter years rather than the more showy emotions.

I dont think he solely uses anger. He uses his own self loathing, physical pain(like you said) and his corruption makes him stronger.
When someone is angry it dosent always result in someone going wild and crazy sometimes it boils calmly beneath the surface. Also he wears a robot suit and you cant see his eyes so vader cant show his emotions visually. When dueling he could have leering, bluging eyes conveying anger or whatever.
I always found that he demonstrates total control. Channeling his negative emotions rather than having them govern him. I think that is what makes him such a powerful force user and dangerous lightsaber duelist.

http://www.imdb.com/list/ls063670214/

I always found that he demonstrates total control. Channeling his negative emotions rather than having them govern him.
That would make him kinda similar to Mace Windu I guess. Though Windu was a light sider obviously. Wonder how close to falling Windu was?

If qui gon didnt die or mace taught anakin vaapad, anakin would have never joined the sith. I dont think mace was ever close enough where it might be possible for him to turn. His mastery of vaapad would never allow him to turn ,he uses his opponents darkness against themselves which is really cool. The ultimate jedi to face palpatine.
Imagine if vader found a holocron of mace teaching vaapad? That would be amazing. I would buy two copies of that comic for sure.
There are similarities between mace and vader, not so much anakin. Especially in their fighting styles.

http://www.imdb.com/list/ls063670214/

Vader gets his power from his emotions, particularly anger and pain. So does the Emperor.

Sure he kills people for their failure and is brutal with prisoners but it's always in a very dispassionate, clinical way. We don't know that he is unfeeling when he does stuff like that. Plus, he is behind a mask. However, we can hear his voice just fine, and it's clear when he is a quite ticked off.

He's using hate and anger, but he's channeling it through terrifying calmness. Vader has been a Sith for 19 years, he has learned what works for him.