Yoda told Luke he did not need more training. That would strongly point to Luke being ready.
Not quite: -
Yoda
No more training do you require. Already know you, that which you need.
Luke
Then I am a Jedi.
Yoda
No. Not yet. One thing remains. Vader. You must confront Vader. Then, only then, a Jedi will you be. And confront him you will.
The implication here seems to be that he has the knowledge and skills to become a Jedi, but it needs to be tested. The old Jedi trials weren't just mindless ritual, they existed for a reason and Luke needed to truly know himself and learn to become truly calm and passive. Vader in the embodiment of all of Luke's darker qualities, figuratively and literally. That's what the cave vision was trying to teach him. He must face the darkness within.
He was not lucky. The screenplay supports what the film clearly lays out--Luke was no longer the weaker participant as in the Cloud City duel:
...and this happens long before Luke gave in to anger. He was Vader's superior at that point.
Again, yes and no. The moment to moment advantage of the fight shifts in both directions over it's course. That's why it's a legitimate fight and not just Vader toying with him as a means to an end as he was on Bespin.
It's complicated though since it all depends how you choose to characterise Vader's state of mind and how it varies throughout these scenes. It's subjective.
At certain points, I think he's genuinely out to kill Luke, while others he's holding back which may or may not have allowed Luke to gain the upper hand at least once. And in at least one instance I genuinely feel Vader is resigned to being replaced. Remember,
he's seen this scene play out before.
Really, you'd have to break down the sequence almost blow-by-blow to really get into what's going on under the surface.
We always need to remember TESB, when the Emperor predicts Luke could destroy them. This has nothing to do with Vader having a change of heart (in fact, he never detected any uncertainty in Vader until well into ROTJ's plot). The force is telling him Luke has the capability to destroy the Sith, and repeatedly acknowledged his growing power. Palpatine never said that about any other Jedi, including the deep ranks of the PT Jedi--the same group of Jedi George Lucas said were the fully trained / height of their abilities Jedi.
Later, Vader repeats the same thing--not as some manipulative tool, but as a statement of fact.
Also remember that Palpatine's weakness was his overconfidence. He foresaw that in one fell swoop he would replace his old apprentice with a new, younger one, finally destroy the rebellion and permanently solidify his grip on the galaxy with his new battle station. Obviously, none of that happened.
It's all academic though since we really don't *know* what version of the future the force actually showed him and how much was his own interpretation. All we know is what he said he saw (which was vague) and he's not exactly known for his honesty and openness.
As for that scene in tESB, there's a lot going on under the surface in that one too. Note how the Emperor seems half stoned. I interpret that as he's been almost completely subsumed in the force. Deep in dark side meditation and ritual. Whatever he saw (Luke destroying him) perturbed him enough to get him onto Vader.
Remember again, we only have *his* word that Luke could destroy they both. Let's say for the sake of argument that what he really saw was Luke turning Vader against him; would he tell Vader this? Of course not. Then on top of all that there's a subtle interplay of word between them, with much being said between the lines.
This is just my personal interpretation, but I think it's a valid one: -
Emperor
There is a great disturbance in the Force.
<You've been busy, working behind my back.>
Vader
I have felt it.
<Busted.>
Emperor
We have a new enemy. The young Rebel who destroyed the Death Star. I have no doubt this boy is the offspring of Anakin Skywalker.
<You have been keeping secrets from me.>
Vader
How is that possible?
<As have you. You told me I killed her.>
Emperor
Search your feelings, Lord Vader. You will know it to be true. He could destroy us.
<Don't bullshit a bullshitter. You're playing with fire.>
Vader
He's just a boy. Obi-Wan can no longer help him.
<You're afraid?>
Emperor
The Force is strong with him. The son of Skywalker must not become a Jedi.
<I know what you're up to.>
Vader
If he could be turned, he would become a powerful ally.
<I can still salvage this.>
Emperor
Yes... He would be a great asset. Can it be done?
<Two can play at this game.>
Vader
He will join us or die, master.
<I'm going to be free of you, one way or the other.>
Perhaps it wasn't his training that was the source of what the Emperor and Vader sensed I terms of Luke being able to destroy them. What if it was something else? The Emperor clearly mentioned that the son of Skywalker must not become a Jedi. There was something there that threatened both Sith Lords greatly.
"Always in motion is the future."
It seems fairly obvious that the threat is exactly what transpired. Luke reached that part of Vader that was still Anakin and thus destroyed both the master and apprentice at once.
Whether he knew the exact shape of that threat is open to interpretation. Something fans always seem to overlook when talking about Palpatine's sprawling, weblike plots and schemes: they almost never go off exactly according to plan. He's constantly improvising and adjusting to adapt. He's very good at it so for the post part, that worked for him. So perhaps his overconfidence led him to believe he could manipulate this wrinkle to his advantage too. He was wrong.
