I appreciate the idea of keeping Vader's identity a mystery between ROTS and ESB, but the real problem with this approach lies in ANH: Obi-Wan refers to Darth Vader as "a pupil of mine, until he turned to evil." If we keep it unclear whether it's Mace or Anakin, this line gives it away. Indeed, no matter how we try to keep it a mystery, then this line will give it away, unless we're somehow able to give Obi-Wan multiple apprentices.
On a somewhat unrelated note, in the same sentence Obi-Wan calls him "a young Jedi named Darth Vader" — but it's established by the prequels that Darth is something like a Sith title. It would be quite possible to remove all references to the title in the prequels — IIRC it doesn't crop up all that often. Darth Maul is named once, and he could easily be renamed Maul or not named at all ("This is my apprentice... he will find your lost ship"). Dooku is only referred to as "Lord Tyranus", and I don't remember where Darth Sidious is named — again, Dooku and Maul tend to call him "Master" and the Neimoidians tend to call him "Lord". I don't mind "Darth" being a Sith title, but it does make Obi's line a little odd. You can explain it by saying that he's simplifying things for Luke and trying not to arouse any suspicions.