Excuse me while I continue to dump long form here:
NPT Vader:
Here is my solution to the Vader problem. I also came up with the idea of an ersatz Vader running around. Someone here convinced me a couple years back that Obi-Wan has to have several people that could be called his "apprentice" for the lie in ANH to work. I also thought that there was no Vader equivalent in LPT since the role is divied up amongst Maul, Dooku, and Grievous and I wanted a more consistent Black Knight. So, I used to think of this guy as the Vader replacement. Something very Vadery without actually being Vader. Something like this guy on the cover of the comic- Tales of the Jedi: The Freedon Nadd Uprising.
But then, like some of you, I thought: What if it was actually Vader? The first surprise of the NPT! Maybe regress the costume along the lines of the original McQuarrie designs a little, but he looks like Vader and is called Vader.
I also thought along the lines of the Dread Pirate thingy, but I didn't plan on spending much screen time on it. Just suggest that Vader has been around for a while and survived many deaths. Anyways, the point of the NPT is to show the Fall of Anakin Skywalker. So we show that. In the process, we see a lot of Dark Jedi and even 2 or more "apprentices" of Obi-Wan fall as well- though somewhat less spectaculary.
The difference is that we see Anakin struggle with it. When he and Obi-Wan fight, Anakin is trying to show Obi-Wan the power of the Dark Side to convince him to join him in using it. Obi-Wan has to show him the power of the light. The fight ends when Anakin goads Obi-Wan into striking him aggressively and Anakin falls into the molten lava or whatever and Obi-Wan is instantly sobered because he thinks he killed his friend in the process.
Anakin is left for dead. His pregnant wife has not given birth to their son, nor does anyone know about Leia. To save her from grief, perhaps Obi-Wan starts the lie then and there and tells Notpadme that Anakin died a hero fighting Darth Vader. The tradegy of Anakin Skywalker is over. And so is Episode 3.
Saying Anakin died and not making Vader's indentity a mystery seems much cleaner to me. Also, to TheBoost's point, it does change the ESB reveal from "Wow! DV is Luke's father!" to "Wow! DV is Anakin Skywalker, fallen hero of the NPT and Luke's Father!" But I think this is still preferable to "Luke is reacting this way because he has just learned, after two movies of us knowing better, that Darth Vader is in fact his father." And by not cutting Anakin out early, we get to see his full fall to the Dark Side. It does set up right away that Ben hides the truth to protect Anakin's memory and loved ones... But I don't think that works against the OT... rather I think it helps smooth out Obi-Wan's character from ANH to RotJ. He's doing it out of friendship to Anakin and Notpadme, out of guilt for his own failings, and out of desire to put Luke on the right path.