I have the answer.
In 1977, Darth Vader and Anakin Skywalker (simply known as Luke's father) were two different people.
Vader did betray and murder Anakin Skywalker. Anakin did want Luke to have his lightsaber when he was old enough. Etc.
They needed a twist, Lucas had that whole "father/son" concept in an early draft so they changed things up for Empire. Good thing too, it's one of the most iconic moments in movie history.
But anyway, as far as Star Wars in 1977 goes, Anakin and Vader were two separate characters. Obi-Wan wasn't lying, Alec Guinness wasn't playing him as if he was keeping something from Luke, and Vader was just that, Vader no relation to Luke.
DiFabio is on the money here.
The original script for Empire Strikes back did NOT have Vader as Luke's father:
https://mishkanyc.com/bloglin/2010/03/27/original-leigh-brackett-script-for-empire-strikes-back/
Now I know what many of you will say: this original script was written by Leah Brackett FOR George Lucas. Lucas didn't like it, so he scrapped it and wrote a new script where Vader was Luke's father. Of course, if Lucas had always intended that Vader be Luke's father, then he would have TOLD Brackett that critical plot point and she would have put it in the script!
Brackett passed away in March 1978, shortly after turning in the script. Even if we give credence to these alleged Prowse interviews from December 1978 where he supposedly reveals that Vader is Luke's father, that still leaves plenty of time between Lucas' rewriting of Brackett's script and Prowse's interview.
It is apparent that Lucas had his moment of inspiration about the father/son thing sometime in early 1978, NOT before writing A New Hope (which wasn't even called Episode IV: A New Hope when it came out. It was just called "Star Wars" when it played in the theaters.)
I have even seen an early version of the Obi Wan/Vader fight where Obi Wan, a non-suit wearing Vader, and Anakin Skywalker fight along the edge of a volcano. Anakin sacrifices himself by throwing himself and Vader into the volcano. Obi-Wan waits anxiously by the edge, to see if anyone will emerge, but then flees when he senses the Emperor approaching. The scene ends with Vader's charred hand rising from the volcano.
If you watch A New Hope, there's absolutely nothing there to indicate that Lucas intended to make Vader Luke's father or the central character in the story. As redlettermedia said, Vader was like a Nazi SS officer. He'd go in and bust things up and let you know the Empire meant business. He was not SPACE JESUS!
He was like an over glorified storm trooper. He was a foot taller than the storm troopers, wore all black instead of all white, used a light saber instead of a blaster. But he still took orders from Grand Moff Tarkin and accompanied general infantry in the space-jet battles. There's absolutely nothing in the first movie to indicate that Lucas intended him to be the nexus of the entire saga.
