This question occurred to me after reading an intriguing question by @Tesserex here that connected Vader's acceptance of death with his ultimate end. This made me think that maybe his strong personality/conviction helped him to pulled through other near-fatal episodes (other than being burnt to a crisp in Ep.III). I haven't read any of the novels, only seen the movies, so all I know about is when he was burned at the end of Ep.III. Just wondering how many times Darth Vader might have been severely damaged or wounded before he ultimately died in Ep.VI?
During his confrontation with Obi-Wan Kenobi on Mustafar, Anakin Skywalker was first maimed by lightsabre [sic] 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 Vader's injuries. Vader may have experienced mishaps between the films. He may have lost more flesh from his limbs before his final fight at Endor. 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 Vader 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 Vader's injuries result from serious burns inflicted when he fell in a volcanic environment
Note that the bulk of the content was written in the mid 1990s; well before the prequels came out.
Vader's back, specifically his spine, was not whole. Vader at some time had suffered serious spinal injury in the upper neck. However, his injuries on Mustafar did not affect the spine.1 This forced Vader to wear a thick electrode-studded collar that supported his helmet to safeguard the cybernetic devices that replaced his upper vertebrae.
However, this newer source seems to corroborate a single serious injury to Vader's spine after the battle at Mustafar.
In the Splinter of the Mind's Eye novel, there was a battle between Luke and Vader 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 Luke, his actions guided by the spirit of Obi-Wan and his power augmented by the Kaiburr crystal, strikes Vader's sword arm, severing it. Undaunted, Vader picks up his lightsaber with his remaining arm, and again pursues the exhausted Luke. Vader, also exhausted, is about to win, staggering as he approaches to make the killing blow, and he falls into a pit; Luke senses that this does not kill Vader.
Every movement of Skywalker was guided by the spirit of Obi-Wan Kenobi and empowered by the crystal, so the young boy amazingly managed to hold Vader off, even managing to sever the Dark Lord's mechanical arm. Shocked, Vader tumbled down a deep pit, ending their duel. This was the first battle between Darth Vader and Luke Skywalker.