Count Doodoo
The Minimalist
http://www.theforce.net/swtc/injuries.html
has this info:
Origin of the injuries.
Little is known about the origin of Lord Vader's infirmities. What seems certain is that they result from grave injuries suffered during his career as Jedi Knight and Sith Lord, between the Battles of Geonosis and Yavin. The damage was inflicted in at least two separate incidents, beginning with the loss of 3/4 of his right arm in combat with Count Dooku. According to chronological information given by George Lucas in official interviews, Vader was only aged in his mid forties when he died aboard the Death Star II; his health problems do not result from old age.
In an interview in Starlog in 1980, Mark Hamill recounts a background sketch which he had been told:
"I remember very early on asking who my parents were and being told that my father and Obi Wan met Vader on the edge of a volcano and they had a duel. My father and Darth Vader fell into the crater and my father was instantly killed. Vader crawled out horribly scarred, and at that point the Emperor landed and Obi Wan ran into the forest, never to be seen again."
This quote is not canonical, but it probably includes some elements of what Lucas considered to be the genuine history at that time. In STAR WARS: The Annotated Screenplays, Lucas reveals that he did hold a lava theory "at one point" but he gives no indication whether it is in the present version of the saga. The story may have changed since then, or important details may have been deliberately distorted or kept from Hamill. At least one major change has been made: Anakin Skywalker and Darth Vader are now identified as the same man. (The second draft of The Empire Strikes Back screenplay tells an alternative story, indicating that Vader fell into a nuclear reactor shaft rather than lava. In this case, his infirmities may have something to do with mutation, radiation sickness and extensive cancer.) The Emperor's supposed involvement is interesting, as is the implication that Vader emerged from the lava under his own strength.
Three years later, aspects of the tale of the molten pit resurfaced in a more official form. In the novelisation of Return of the Jedi, Obi-Wan Kenobi told Luke Skywalker:
" We fought ... your father fell into a molten pit. When your father clawed his way out of that fiery pool, the change had been burned into him forever --- he was Darth Vader, without a trace of Anakin Skywalker. Irredeemably dark. Scarred. Kept alive only by machinery and his own black will ... "
Later, when Vader lay dying he recalled those painful events:
These were memories he wanted none of, not now. Memories of molten lava, crawling up his back ... no.
This boy had pulled him from that pit --- here, now, with this act. This boy was good.