Oh, well, in a happy threesome AU, the trio would be a well-oiled functioning team so as soon as they landed on Vader’s ship, they’d get the lay of the land and realize that something’s Not Right. When Vader sees them, he probably thinks they’re a hallucination or a dream or something. Here’s two of his favorite people, both alive and within reaching distance, but they’re with a version of him that’s not in a suit, who looks healthy and happy? This obviously must be a dream.
The happy trio kind of go with it.
Until one of the stormtroopers come along and freak out about the strangers on board and Vader realizes that they’re real and has them arrested. He can’t bear to throw Padme into a cell, though, so he orders them all put into one of the guest rooms, though shackled and under heavy guard.
Anakin’s the first to figure out that Vader is his evil twin and he’s determined that the way out of this is to appeal to the more logical members of their trio: this universe’s Padme and Obi-Wan.
So the next time Vader comes to try to intimidate them into answering his questions (it’s not going anywhere), Anakin says “We want to talk to Obi-Wan and Padme. Where are they?”
Vader explodes and throws Anakin against the wall and is like “Padme is DEAD.” Then he watches as this Padme - this miraculously alive and well Padme who’s not his, who’s got some white mixed in within her brown hair now, older than his own Padme ever got to be (because of him) - rushes over to the other version of himself and he both envies and loathes his other self, for still having what he’s lost.
“I’m alright,” Anakin reassures Padme from where he’s collapsed on the ground. Vader wonders if this is what he would look like if he wasn’t confined to his suit: older, hair starting to go gray at the temples, wrinkles around his eyes but still light on his feet.
“What do you mean, Padme’s dead?” Obi-Wan asks and he’s still got that cultured accent, that lilt in his tone. He’s got a head full of white hair but he holds himself straight and tall and he’s still in top fighting form. If Vader stares a little too hungrily at him, well, the mask is good for some things. He wonders if that’s how the Obi-Wan of this world looks like now.
“Hey,” Anakin snaps. “Eyes above the waist, buddy. You can ogle your own Obi-Wan.”
Vader snarls. He’s had enough of these memories of things he could have had. He stomps out of the room without another word.
He can’t stay away though, not really. Not when he never thought he’d see Padme again, not when his own search for Obi-Wan has been so frustratingly fruitless. So he does go back. In a couple of days, after a particularly vexing holocall with his Master.
This time, it’s Padme who speaks first and she says, “We want to help you find Obi-Wan.”
Vader rears back in surprise. “What?”
“We’ll help you find your Obi-Wan,” she repeats. “You said that I died in this universe, but you didn’t say anything about Obi-Wan, so he’s still alive, right? But Anakin and Obi-Wan can’t find him on board this ship and going by your….well, your everything, the two of you must not be in contact so either you’ve never met or you’re separated for some reason.”
“He betrayed me!” Vader says. “He left me to die on a volcano and he put me in this suit! I’ve been looking for him for almost two decades and haven’t found him. He’s a wily old coot!”
“Well,” Anakin says with a wry look at his own Obi-Wan, “he is that.”
Obi-Wan rolls his eyes.
“Luckily,” Anakin continues, “we have an Obi-Wan here who can help us figure out where your Obi-Wan went. Since they’re basically the same person.”
“With vastly different life experiences, though,” Obi-Wan cautions.
“Depending on when the divergence happened,” Anakin says. To Vader, he continues, “But in order to do that, you’ll have to tell us everything that’s happened.”
“Why would you help me?” Vader asks suspiciously.
“Because this world is fucked up,” Anakin says bluntly. “And we want to get home and Obi-Wan’s the smartest person I know, especially when it comes to Force-related shenanigans. He is the Master of the Order, after all. So the Obi-Wan of this world would be our best chance of figuring out how to fix this. He might also be able to help you fix all this, too.” Anakin gestures at their surroundings but Vader knows he’s including the whole universe in that.
Vader thinks back on the past two decades under Sidious’ rule, thinks back at the transmission from his Master ordering him to go to Scarif, to chase after a couple of Rebel infiltrators.
He scoffs, internally. He knows what the Rebels are after; they know that there’s a weakness in the construction of the Death Star. But even if they can pinpoint it, there isn’t anyone who can hit it. No one besides him.
Let them get the plans. He’s going after bigger fish.
“Okay, then, old man. Let’s see how much like Obi-Wan you really are,” he says.
Obi-Wan just arches his brow at him and oh, that brings back memories.
“Once again, eyes above the waist, buddy,” Anakin says, this time with just a hint of a smirk.
Vader wants to kill himself.