I have always chalked the mystery loss of that part of his one arm up to either a battle injury or some issue with the prosthesis on that arm. The dismemberment appointment in Lost Command look painful, but since he goes back into the field almost immediately, I don’t think it was necessarily damaging, or damaging every time- that would be a bad way to maintain an Imperial military asset which is about to go do some more I Am Become Death shit.
There are honestly a lot of ways to fuck up a limb or joint badly enough to lose part of it- a lot of which don’t even involve accidental mechanical damage. Somebody- not sure if it was @jerseydevious or @darthnickels- once suggested that “Sith alchemy” was just straight-up steroids, and prolonged steroid use can cause avascular necrosis (bone death, sometimes with destruction of articular joint surfaces). So can trauma, like from a surgery or injury. So can vascular compression (like from shitty prosthetics or internal cybernetics) or a blockage like from a clot. So can radiation (like from being in space 24/7, though I imagine that they’ve figured out how to deal with that in Star Wars, otherwise like 1/3rd of any given space navy would get cancer or at least cataracts). So can autoimmune issues. He could also have just gotten an infection from the initial amputation.
I think you can write it pretty much any way you want, there are just a lot of ways for Darth Vader to lose bits.
(people who follow my blog: first, I’m sorry. Second, I’m tagging this kind of content as “body horror,” “gross medical stuff,” and “darth vader body horror,” and will do so consistently.)