I have been watching the final duel in ROTJ and my assesment is that Vader is not really fighting: he's giving up.... I mean the dark lord of the sith "fell" and cant get up...

In ESB Luke did not know till after that hes his father and as lucas said in a Interview concerning the original trilogy he thinks he is fighting the man who killed his father

On Bespin Luke is battling with all his might not knowing who he is fighting., cut to Endor, this is the fight... ...

Luke is fighting but holding back a little and just defending himself until that moment that vader find out about Leia

The powerful Choir music plays...

Luke is enraged that vader has endangered the life of his sister then overrun with emotion Luke broke the Code and starts beating vader down till off with his hand.. as I said before it looks like he is giving up, does Anakin know its near the end for the empire?

Has luke reached him before the helmet comes off.. is this why vader then slowly is defeated but saves the life of his son before he dies.....Does Luke go to the dark side in that final duel?? any way you look at it.... Vaders Final duel does bring balance to the force... all in all its the best fight in the saga bar none