Did a little searching online and found this quote from the ROTJ novel,

" 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 ... "

I also found that in a pre-Empire Strikes Back interview with Mark Hamill, he mentions that he was told his character's father and Obi-Wan fought Darth Vader over a volcano, and both Luke's father and Vader fall in, with Vader managing to survive.