I read that Star Wars is "just" the tragedy of Darth Vader or Anakin Skywalker. By Episode 7 Vader is long dead. How does Force Awakens fit into this bigger picture? Is it the start of another 6 episodes about Kylo Ren?
The very short answer is that the new trilogy is highly unlikely to fit in with the 9 episode arc that was originally envisioned by Lucas. George Lucas' scripts, story ideas and outlines for Star Wars 7, 8 and 9 were comprehensively rejected by Disney and the new trilogy can be seen to be largely aping the original trilogy with the very clear intention of spawning a new franchise, one that is (presumably) going to be based around Kylo Ren's ultimate redemption at the hands of a new generation of Jedi.
Given that Vader's entire substance in the latest film was a fleeting mention that he's been a role model for Ren and a couple of throwaway scenes with his hat, Anakin's involvement in the subsequent films is peripheral at best, bordering on non-existent.
The 'Saga of Darth Vader' is almost certainly at an end.
First off, I'm having difficulty finding anyone officially claiming that Star Wars is just about the tragedy of Darth Vader.
While Lucas claims that the series is about Vader, he also claims that the original three films aren't tragedies.
Yeah, I made a series of movies that was about one thing: Darth Vader.
The first three episodes are a tragedy, and the second three go slightly goofy, but they're inspirational
However, if you believe in the tragedy of Darth Vader, The Force Awakens fits in very well, perhaps unintentionally.
In Return of the Jedi Anakin rejoins the Light Side and becomes a Jedi again.
Luke (to Leia) Because...there is good in him. I've felt it. He won't turn me over to the Emperor. I can save him. I can turn him back to the good side. I have to try.
Vader: You already have, Luke. You were right about me. Tell your sister...you were right.
He looks off to the side and sees three shimmering, smiling figures at the edge of the shadows: Ben Kenobi, Yoda, and Anakin Skywalker.
However, in The Force Awakens, the Vader persona he had previously thrown off is heavily idolized by Kylo Ren.
Kylo Ren misunderstood him so much that he murdered younglings (and committed various other atrocities) in his name.
Ren: Grandfather,... I will finish what you started.
That's pretty tragic.
While the Saga of Anakin Skywalker is at an end, the results of his misdeeds are still being felt around the galaxy. You may find it ironic that, for example, his behaviour is still having an effect on his family -- specifically that it would lead his grandson to the Dark Side (and the betrayal of his family).
So while Anakin's tale may be over on a personal level, the galaxy hasn't quite recovered yet.