In the opening of the novel, Nick reveals that it was "foul dust" that floated in Gatsby's dream not the dream itself which destroys him. Nick also understands that Gatsby turned out all right in the end. Even in death, Gatsby still clings to the elusive hope of Daisy. Nick turns his anger toward the "careless people" of the world like Daisy and Tom who destroy dreams but remain in their bubble protected by their own immorality. Therefore, Nick did understand Gatsby's vision.
All through out the novel, we see Nick learning more and more about Gatsby and about his vision. This does not happen all at once but in a series of events. Initially, Nick hears Gatsby's parties next door to him but it is not until he attends one himself that he realises that all the people Gatsby surrounds himself with do not care about him. He is alone and lonely even though he is surrounded by people
As the novel progresses, we see Nick becoming more and more aware that Gatsby's persuit of Daisy is what motivates him in all decisions. He has the house and the parties to lure Daisy back into his life. He is separated from reality. I think the pinnacle of Nick's understanding comes when Gatsby confides in Nick that it is possible to change the past. I think in this exchange we see Nick coming to understand the entirety of Gatsby's fictions, his hopes and his dreams.