Within the OT alone, GL always conceived of Vader as a character who fell into a volcano and gets burned even before ANH.
It’s about Ben and Luke’s father and Vader when they are young Jedi knights. But Vader kills Luke’s father, then Ben and Vader have a confrontation, just like they have in Star Wars, and Ben almost kills Vader. As a matter of fact, he falls into a volcanic pit and gets fried and is one destroyed being. That’s why he has to wear the suit with a mask, because it’s a breathing mask. It’s like a walking iron lung. His face is all horrible inside. I was going to shoot a close-up of Vader where you could see the inside of his face, but then we said, no, no, it would destroy the mystique of the whole thing.
So I guess it would make more sense for his eyebrows to be burned off.