Ultimately, Star Wars is made by a lot of authors with a lot of different visions and external pressures. I’ve heard three main interpretations:

• Bringing balance to the Force means destroying the Sith, because darkness is inherently evil. Vader fulfills this by killing the Emperor and becoming redeemed in his last moments, “destroying” the last Sith. (Ignore the whole fleet of off-screen Sith retroactively placed there by Expanded Universe works.)
• Bringing balance to the Force means making the number of Jedi and Sith equal, and Vader fulfills this by killing hundreds of thousands of Jedi.
• Bringing balance to the Force means destroying the black-and-white view of Jedi and Sith, which definitely seems to be the position of the Bendu (an ancient loosely-all-knowing alien who shows up in a handful of episodes of Clone Wars, but has been treated by the broader franchise as disproportionately important). Vader fulfills this by breaking the boundary between Sith and Jedi in his dying moments—or maybe the prophecy was about Rey after all. (Doesn’t this make the shot of Luke’s face inside Vader’s helmet the most interesting? Almost as though it’s saying both “there is evil in you, but there is good in him too.”)

Which is “right”? Well, you’d have to ask Lucas, wouldn’t you? Or Rian Johnson, or Dave Filoni, or the entire writing team of Knights of the Old Republic, or any of the thousands of people who’ve written for Star Wars… There’s not just one answer.

I think any of these could be true, from a certain point of view.