re: Another Star Wars: Doesn't episode 7 nullify Anakin/Vader as "The Chosen One" ?

Balance doesn't mean no dark and no light. I believe you were the one asking about the clone wars series the other day. It has a three episode trilogy in season 3 which explains what exactly balance means. For there to be balance there must be dark and light, you cannot have one without the other. The problem was both the Jedi and Sith were using their preferred side to try and manipulate the force and tip the balance in their favor.

There are more force using groups than just the jedi & sith. Balance goes beyond just them. They were the two parties that were both guilty of the imbalance. Jedi through their dogmatic views and use of political influence and the sith for obvious reasons

For Anakin to bring balance, he had to purge the Jedi of their dogmatic views first before he eliminated the Sith. Balance was finally achieved when no active force user group was able to tip the balance too far in either light or dark direction.

So, no, 7 does not nullify that. Do I expect Abrams to be lazy and make a "new" chosen one? Absolutely and I will be very disappointed when it happens.