Vader didn't want Luke to die. However, that's doesn't mean he wasn't even trying against Luke. No, he was trying to perform Cho Sun or get him a surrender position, which he was incapable to do throughout the duel in ROTJ and ESB. This shows Luke isn't a newbish Jedi, but very strong for a person of his stature. Yes, he can definately take on QGJ. ESB Luke is not better than Jinn, but ROTJ Luke is definately on par or even better.
Now Plo Koon is stronger than QGJ, therefore he beats Luke. Howver, he doesn't own Luke in thirty seconds. I would say Skywalker dies after a couple of minutes's of useless determination.