The main idea of a story is the theme, message or point. It is what the author is trying to tell you. This clever story is trying to tell us that the person telling a story controls the perceptions of the audience. The main point is that a story will change depending on whose point of view it is told from.
Interestingly, this is Vance’s point. He is trying to get his students to understand the importance of point of view, by intentionally biasing the story. However, that is not even his purpose. His real purpose is to get one of his students to fall in love with him.