About the prophecy bringing balance: it was Luke who brought balance, not Anakin.
Really? Answer this. Who killed the Emperor?
Anakin "brought balance" by killing the Sith Lord, not Luke. This was one of the FEW good things in the whole set of movies--that Anakin was, indeed, the "chosen one" but the problem with vague "prophecies" is that they are VAGUE. It was a pretty subtle commentary on the perils of religious faith. I'm sure it was a mistake on Lucas' part, but it was a good one.
While I do understand that the universe with Jedi in power is "balanced" by bringing the Sith to power that's not what balanced it, the Sith were always there. Luke used his emotions and gave in to both the dark and the light at times without being fully swayed by either. He gives in to light more with all the training and everything but still rejects all of it when he leaves Dagobah and leaves his training against the Jedi master's dogma because of his emotional ties to his friends on Bespin.
The whole "balance" thing was an subtle, but very obtuse plot line. Had Anakin NOT been born, the Emperor would have taken over anyway. He would have wiped out the Jedi, anyway. This was the "imbalance" the prophecy was talking about. That's why the Jedi were all willing to take a chance, remember all those comments about how "blind" they were? So even without Anakin, the Jedi all die and the universe is plunged into darkness, a supreme imbalance. At least with the Jedi, some evil is tolerated--like slavery, smuggling, drugs. The Jedi upheld the balance even when they were in power. The Emperor though was fine with ultimate acts of evil, genocide ect.
So, whether Anakin was there or not, Emperor wins. And imbalance is had.
However, had Darth Vader not existed, no one would have killed the Emperor. Without Vader "reigning" in the Emperor and asking him to try and turn Luke, he would have been killed back in New Hope. Or surely after that (Vader's the only reason the Star Destroy doesn't blow the Millenium falcon up at the end of Strikes Back, for example.)--half the time when Luke survives it's because his father stopping him from being killed in his futile quest to "turn him to the dark side".
And just to put a massive and ultimately symbolic cherry on top of this philosophical train ride--Vader ACTUALLY physically throws darkness back down into the pit it came from. So, he is the "chose one."--the problem was the prophecy never spoke about when he'd save everyone, who even what save was. Like I said, I'm sure this subtle element was a completely mistake on Lucas' part. After seeing the prequels, I know he has an idea of the thematic/symbolic elements he wants in his stories, but he's SO bad at implementing them.