"I'll never join you!" Luke shouted, trying to push past the shock that gripped him and the pain of his missing hand.
"If you only knew the power of the Dark Side," Vader replied, implacably. "Obi-Wan never told you what happened to your father."
"He told me-" Luke began, then stopped. "I – wait… there's… there's a dark secret here, isn't there?"
He winced, trying to focus. "I… hold on… I think I get it."
"I doubt that," Vader said.
Luke wasn't really listening. "That actually explains a lot," he said. "The whole… all the contradictions. The navigator on a spice freighter thing, the… ace pilot and Jedi thing… and, why you'd…"
"Search your feelings," Vader said. "Obi-Wan never told you-"
"I don't know if you or Obi-Wan are the ones deadnaming him!" Luke said. "Or her, but – I know what happened now!"
"What," Vader said, because this conversation wasn't quite going the way he was expecting.
"You betrayed and murdered my mother!" Luke shouted.
Another gust of wind came, and he spent the next several seconds trying to hold onto the gantry and avoid falling off into the depths of Cloud City's ventilation system.
When he looked up, suddenly worried, he saw that Vader wasn't advancing towards him with an implacable ruby blade held high. Wasn't reaching out with the Force, to throw detritus at him.
In fact, Vader appeared to have forgotten to breathe.
"...well?" Luke asked, feeling giddy, which was probably also the shock. "Don't you have anything to say for yourself?"
Vader's respirator let out a long, high-pitched whine, which could be an expression of some intense emotion too great for a mortal frame to bear or could just be because he'd thoroughly violated the warranty.
"I… didn't mean to?" he said.
"You didn't mean to?" Luke demanded. "How can you not mean to betray and murder someone?"
"I was titanically sleep deprived," Vader stated. "I literally do not remember the journey from Coruscant to Mustafar. I kept dreaming about her death, I'd have done anything to stop it happening."
Luke managed what he thought was quite a good incredulous stare, considering.
"Anything?" he asked.
"Over throw the government," Vader clarified. "Turn to the Dark Side. Pledge my life to the Emperor."
"And you killed them anyway!" Luke objected.
"Did I mention the lack of sleep?" Vader asked, his voice definitely hurt. "I do not know what the medical term is for being so low on sleep you try to choke the most important person in your entire life, but there should probably be one and it should probably be named after me."
He let out a long, rippling sigh.
"I just… I just… I can't face this now. I've got too many emotions today. I'll do this confrontation again later when I've worked out how to put it."
He turned, and walked away.
"...um," Luke said, not sure what to do now, then looked at his remaining hand.
On balance, he was pretty sure he could fly an X-Wing one handed…
AN:
Leaping to the right conclusion, the wrong way around.