Yeah Vader was supposed to be one of many Dark Lords. As per the Lucas/Foster story conference on Splinter, and in the novelization of Star Wars. That part was never edited out of the book itself but the audio tape abridgement changed it to just Vader being the Dark Lord. in the 1990s. John Whitman did the edited abridgement.
In the rough draft, there’s a bunch of “Sith Knights”, with “Prince Valorum” being the only one to be named and “Darth Vader” is just a general. In the 2nd draft, “Darth Vader” is now a Sith Knight and is a “Lord” (but none of the other Sith Knights are) and his master is “Prince Espaa Valorum”. In the 3rd draft, all the Sith are “Dark Lords” instead of “Knights” and Vader now serves the Emperor directly. In the 4th draft and revised 4th draft, Vader is the only “Sith”.