I would say that the diffirence between Vader and Greivous is that even with being rebuilt in that suit Vader had to rely on himself, his human self, and possibly the force to be able to get about. If Vader was in his prime, as Anakin, he would have beaten Luke no problem.
There's another possibility. Vader isn't a monster, at least compared to Palpatine, Tarkin, ect, he's still grieving over Padme (a comic shows the inside of his meditation chamber we see in ESB). And stopping Luke from killing Palpatine, defending the Emperor or preventing his son from falling to the dark side? Maybe, just maybe, Vader drags his ass in doing the Empire's dirty work, maybe more so in a situation such as facing his old master.