It wasn't Vader. It was a clone.
I don't think the Vader that Starkiller fought and captured on Kamino was Darth Vader/Anakin Skywalker.
I think it will be revealed that it was actually a clone of Vader.
We saw that Starkiller dropped Vader with Force Lightning and basically rendered him useless in the beginning of the game - yet "Vader" stood up to a relentless barrage of Force Fury lightning on Kamino. Maybe the life support suit that is known to not handle electricity so well was actually more of a costume for a Vader clone.
Also...Vader captured? Really? I don't think that even in these days of Star Wars expanded universe garbage would Lucas/Lucasfilm/Lucasarts allow the big bad villain and central focus of the entire saga just be neutered and captured.
Perhaps a clone of Vader (we know that clones of Jedi can use the force) was used as another ploy by Vader to lure Starkiller again without having to risk being killed or even...captured? Vader now knows exactly where the Rebellion is going - exactly where Starkiller, Juno and Kota are going and where a supposed "hidden Rebel base" is.
Yea, I am just trying to figure out a reasonable explanation for the down right strange ending of TFUII...