As far as I can determine, it appears that George Lucas always intended to depict Vader as a human. This can be seen in the Dark Horse Comics miniseries The Star Wars, which is based on the original draft of Lucas' script for what eventually became A New Hope.
Here is some concept art for the character's depiction in The Star Wars, the comic:
There were some other notable changes to the character from the original draft, such as the fact that he originally wasn't a Sith, but a General in the Imperial military:
You'll also notice in these images that Vader isn't as cybernetic as he is in the final films. Lucas originally did not envision him as a cyborg. That came in a later draft of the script, as described here at starwars.com:
“Early in the script there was a description of Vader crossing between two ships in space so I created this mask so he could breathe in space, with a suggestion of teeth in the masks [sic] grillwork,” McQuarrie said. It was not until the fourth draft that Lucas wrote in more of the backstory of Vader, with his connection to Ben Kenobi and Luke’s father, as well as the duel that followed between Ben and Vader after Vader killed Luke’s father. The duel ended with Vader falling into a volcanic pit and needing the suit and mask that Ralph McQuarrie had come up with.
So, while Vader did undergo some changes between his inception to his ultimate depiction in the Original Trilogy, there doesn't seem to be anything to indicate that Lucas ever planned for him to be a non-human character.