Okay so he could have learned there, but it couldn't have been much... Because of the time in between the movies and how hard it is to train like that.
Now on that thing you said, its not that Luke pushes Vader on the defense, its that Vader is on the defensive that makes me think he isn't trying. Luke although skilled in the force is no where near Vader in lightsaber skills or force power. Where Vader would have just walked up to that ship and blown the entire thing up Luke had to work.
Now seriously Vader wasn't trying to kill Luke yes he could have defeated him without killing him but would that have done? Get Palpatine to kill him because Luke wasn't worthy... Luke his skill in that fight although he attacks is worthless. Lets look at it like this
A normal guy can attack a skilled fighter with a sword the skilled fighter should be able to kill him but if he doesn't want to he still has to block and avoid, however would his opinion change and would he want the other to die he should be able to do it.
Luke his fighting method isn't all that good. He just swings... So ROTJ Luke is not a fighter, he understands what Jedi are about and he is a Jedi in that sense of the word, but is he a fighter? No. Would he have defeated Vader if he was not his son? No. Could Vader have killed him? Yes. Did Vader want to kill him? No.