It's harder to give a "proper acting" when you have a helmet hiding your subtle eye movements. But the character arc is there if you pay attention. Vader comes from finding his son is alive and just blew the Death Star, then proceeds to find him and try to make an ally out of him against the Emperor. Right here, he's already trying to screw over Palpatine (the emperor orders Vader to kill him, Vader convinces him he would be a "great asset" to the Dark Force). In retrospect we get that Vader was trying to ally with Luke to destroy the Emperor and rule the galaxy, while Palpatine was trying to arrange a fight between those two and he would stay with whomever would win.
In ESB, Vader is pursuing Luke like a madman. He sacrifices Star Destroyers within an asteroid field just to get his friends to build a trap for Luke. He confides his fatherhood. But at that last moment, before Luke escapes, you do see a psychological back and forth between them. There's a connection between them. That builds Vader's character much further than anything done in ANH. When Luke escapes, he manages to not choke anyone, or being angry. He just appears sad to see his son going away. And you get that despite his helmet. I think that's actually not bad acting.
In RoTJ we see the conflict within Vader. When Luke surrenders to him and talks to him, you do see him wrestling with himself. Again, all within the helmet. "It is too late for me", that's quite the sentence he gives, quite the hint that he is not just this evil beast. Then he gets his feet on the ground again and tells him the emperor will show him the true meaning of the force. Luke tells then that Ben was right after all, his father is dead. And Vader just wrestles again, silently, after he's gone up. That's character development.
While fighting Luke, Vader seems more in control with himself, but that's just because the emperor is looking. Yes, it only comes to the surface and to the action right at the last minute, but the conflict was there and was acted. It showed. If you didn't see it, I recommend you watch the movies again.