Darth Vader wasn't originally going to be a suited cyborg. This was created when the concept artist drew the opening scene where the ship was being boarded. It was initially imagined that Darth Vader would fly through space to enter the ship, necessitating a suit and breathing mask. This was later made permanent and incorporated in the story.
It is also worth noting that "Vader" is the Dutch word for "father", and it may be tempting to read the character name "Darth Vader" as "Dark Father", a word-play that may well be the root of the Sith title. However judging by the origin of the other Sith names, Vader may also possibly be a derivative of the word invader. This issue is matter of a long debate in the official Star Wars Forums.
