If Vader is was so strong he would have taken on Palpatine on his own (Palpatine really is strong and hides his great powers behind a mask of being a sick old man, he is the counterpoint to Yoda who hides his power in much the same way).

Vader and Palpatine are at a point of stalemate with each other, they both want the other out of the way and see Luke as the tool for doing it.

In the end it's Anakin's latent ability to feel compassion that makes him strong (even stronger than Palpatine).

There are two Vaders here, the one he wants us to see, the Dark Lord who crushes all who stand in his way and the physically and emotional wreckage that sits inside the armoured shell.

He hasn't had to fight a Jedi in decades (leaving the EU to one side and allowing for the obvious fact that Obi-Wan allowed himself to be killed). So this is the first time he has had to extend himself in this way for a long time.

Luke loses this battle but he puts up a strong fight (with only a few days training and youth on his side) which proves just how weak Vader is becoming.

So I'd allow him a few grunts and grrrrs of anger and pain.

Of course Vader pales in comparison to the Emperor. Not by too much. The only card the Emperor can play against Vader is Force Lightning. That's not to say Vader is weak in contrast to everyone else he's around. The Emperor is the only exception, which is saying a lot about how much power Vader has on his hands.

You're right in that he's most likely been out of the Jedi dueling business for a while. However, I see no reason to not believe, and every reason to believe that he's consistently at the forefront of war and conflict. I'm just saying this guy is hardened to pain and his reactions are weird and out of place.