Violet remains somewhat irritating throughout the novel, but has the important distinction of being the first person (outside of Dick, Tommy, and her family) to see Nicole’s mental illness. When she does see it, she wants to tell everybody about it. When Tommy stops her it provokes the duel between Tommy and Albert. The duel seems insignificant at first, but we later learn that Albert thinks the duel (which he didn’t back down from for fear of losing face with his wife) helped transform him and make him the success that he is.
Like her husband, Violet is a meta-fictional character. She knows full well she’s inside of a book. When she first meets Rosemary, she tells her "We thought maybe you were in the plot. […] We don’t know who’s in the plot and who isn’t." Rosemary is confused. She doesn’t know she’s in a book. She thinks that she at the beach, silly girl.
But Violet is actually a little confused and so is her friend Mrs. Dumphrey. They think they are readers. They think they are being excluded from the book. Mrs. Dumphrey tells Rosemary, "We’re not in it. We’re the gallery." Then just to sink it in, after McKisco has told her to shut up about it, Violet asks Rosemary, "Going to stay all summer? If you do you can watch the plot unfold." Albert’s subsequent outburst is hilarious, once we get the joke: "For God’s sake, Violet, drop the subject! Get a new joke!" he says. Though funny, this is also Fitzgerald through Albert McKisco self consciously expressing doubt about his own literary techniques.
We’ll leave you with a question: when Violet tries to talk about what she saw in the Divers' bathroom, is she trying to force her way into the plot? Does Tommy keep her out when he won’t let her tell what she’s seen?