I see it this way:
ANH: Vader (together with Tarkin) is the villain who killed Luke’s father and his mentor.
TESB: Vader captures and tortures Luke’s friends to get him and later reveals to be his own father.
ROTJ: Luke is said to defeat Vader, yet their conflict relationship brings them together and overthrow his master.
To me, it’s all about their relationship, one way or another.
Those first two examples aren’t really a relationship. There’s no real back and forth. They don’t interact directly at all in the first movie, only briefly interact (albeit dramatically) in the last act of the second movie, and Luke, it can be argued, doesn’t really know WHO that guy was that killed Ben by the end of Star Wars.
The third movie is where any actual relationship really happens, has an arc, and resolves. The very last 20 minutes of the second movie sets up a relationship that COULD happen. But again - you can’t really describe the first 2/3rds of Empire as a “relationship.” They don’t interact. Luke doesn’t even know who Vader IS.
And again - Vader being Luke’s father was never any part of a plan. It was introduced very late in the game and almost specifically to deploy a cliffhanger so out-of-nowhere that audiences would feel compelled to see the third movie to see how that resolved.
Spoilers for new Star Wars movies are always pretty revealing in how so much of what we enjoy about Star Wars are things we enjoy because we only ever saw the final product sans any “discourse” that consistently surrounded all aspects of production. Things we take for granted as great storytelling are often taken as such because we hadn’t yet infected ourselves with a veritable rulebook of mythology that we can’t wait to break out whenever we hear about stories that we haven’t seen yet, to pre-emptively show how deeply in violation they are.