Han got the first shots in on Vader on Cloud City. And what did Vader do? Just absorb the blasts into his hand.
And Vader is "weaker" because of his injuries.
If I remember correctly, his gloves are of a certain material that blocks laser shots?
But if he could get Chewie to remove the arms...
And then I get Force Unleashed flashbacks of Vader choking/killing any wookie for fun and in FU2 DLC that Starkiller's clone killed Chewie and Han for fun. At least FU is cannon, FU2 DLC certainly not.