During his confrontation with <PERSON> on Mustafar, <PERSON> was first maimed by lightsabre cuts. He lost both legs around the knees, and much of his left arm. He fell onto a slope of volcanic sand, and slid helplessly to the edge of a lava river. His body ignited due to heat radiated off that flow. His lungs were scorched by the hot gases he inhaled. [Revenge of the Sith novel; graphic novel; childrens' novel.]
This explains most (but not all) of Lord <PERSON>'s injuries. <PERSON> may have experienced mishaps between the films. He may have lost more flesh from his limbs before his final fight at Endor. 605
His spinal damage may have resulted from an accidental fall into a pit on Mimban [in Splinter of the Mind's Eye, though the book doesn't show how badly he was hurt]. Lord <PERSON> could have suffered extra injuries during his hunts for surviving Jedi and the Clone Wars campaigns that continued after the Empire was proclaimed.
Nonetheless, if the passages from Return of the Jedi are taken literally, the worst of <PERSON>'s injuries result from serious burns inflicted when he fell in a volcanic environment
http://www.theforce.net/swtc/injuries.html#origin
Note that the bulk of the content was written in the mid 1990s; well before the prequels came out.
<PERSON>'s back, specifically his spine, was not whole.[9] <PERSON> at some time had suffered serious spinal injury in the upper neck. However, his injuries on <PERSON> did not affect the spine.[1] This forced <PERSON> to wear a thick electrode-studded collar that supported his helmet to safeguard the cybernetic devices that replaced his upper vertebrae.
http://starwars.wikia.com/wiki/Darth_Vader's_armor#Spine
However, this newer source seems to corroborate a single serious injury to <PERSON>'s spine after the battle at Mustafar.
In the Splinter of the Mind's Eye novel, there was a battle between <PERSON> and <PERSON> at Mimban, but the summaries seem to conflict somewhat. However, after skimming my copy of the novel, the summary from the novel is correct.
They continue to battle and <PERSON>, his actions guided by the spirit of <PERSON> and his power augmented by the Kaiburr crystal, strikes <PERSON>'s sword arm, severing it. Undaunted, <PERSON> picks up his lightsaber with his remaining arm, and again pursues the exhausted <PERSON>. <PERSON>, also exhausted, is about to win, staggering as he approaches to make the killing blow, and he falls into a pit; <PERSON> senses that this does not kill <PERSON>.
http://starwars.wikia.com/wiki/Splinter_of_the_Mind's_Eye
Every movement of <PERSON> was guided by the spirit of <PERSON> and empowered by the crystal, so the young boy amazingly managed to hold <PERSON> off, even managing to sever the Dark Lord's mechanical arm. Shocked, <PERSON> tumbled down a deep pit, ending their duel. This was the first battle between <PERSON> and <PERSON>.
http://starwars.wikia.com/wiki/Mimban 605