What is vision?
"Vision" is a term swiped from the art world - hey, writing's an art form, too! It stands for the direction in which you see your work going, or the effect you want to convey to the reader. What do you want to say with your manuscript? What do you want to stay with the reader once she closes your book? What impact should it have?
Tall order? Yep. But pretty easy to fill, as long as you are consistent in your plot and in your characters' motivations. Your reader will follow right along with you, as long as you're clear about your vision from start to finish. That means sticking to it, even if someone else disagrees with it (and there's a big difference between a disagreement over writing mechanics and your underlying vision - good to remember). Only you can tell your story, and only you should.