The Ryder Windham novelization of Anakin's / Vader's life aimed at a younger audience (The Rise and Fall of Darth Vader
) implies that Obi-Wan was taunting Vader aboard the Death Star by not calling him either "Anakin" or "Vader" during their battle. Vader's thoughts during the course of the fight are that his former Master was trying to irritate and confuse him by not properly using his Sith title.
The real world explanation is most likely that, as far back as 1977, George Lucas had yet to settle on whether or not "Darth" would be a first name or a title given to Vader. Lucas can say all he wants about having a fully fleshed out backstory written that early in the history of the Saga, but we all know quite a few of the early names were different from what they ended up being on screen (Deak Starkiller, for instance).