How is Mathilde from "The Necklace" vain? Thanks.

From early on in "The Necklace," Maupassant defines Mathilde Loisel's personality in terms of her vanity. She is is a person who cares deeply about appearances, about wealth, status, and glamour. There's a tension between her own exalted self image (the kind of life, Maupassant tells us, that Mathilde believes she has been born for) and the actual reality of her current condition, and this results in great distress and even shame.
She exhibits this fixation on appearances throughout the events of "The Necklace." When her husband first tells her about the party, her reaction is actually to refuse attending, on account of not having a suitable dress, and after her husband buys her a dress (one which cost 400 francs, which is established by the narrative as a quite significant sum), she next complains about the lack of jewels. Take careful note of some of her complaints here and the focus she places on appearances (specifically the appearance of wealth) and the perception of others. At the thought of not having jewels, she complains, "I shall look poverty-stricken. I would almost rather not go at all." She further states that "there's nothing more humiliating than to look poor among other women who are rich." This vanity, then, is not only established through way of the story's narration (Maupassant's description of Loissel) but also by Mathilde's own statements and actions within the story.

Mathilde is vain because she thinks she is better than she really is. Although she leads a pretty normal, lower-middle class life, Mathilde genuinely believes herself to be of noble descent. When her husband receives an invitation to the Education Ministry ball, she sees an opportunity to show everyone just how special she thinks she is. This leads her to borrow what she falsely believes is Madame Forestier's valuable diamond necklace for the night.
At the ball, Mathilde is the center of attention; all the men want to dance with her, and the women comment on her incredible beauty. Mathilde is in her element. At long last, she feels truly at home in what she sees as her natural habitat: a high society gathering attended by the social elite. Mathilde loves showing off the necklace to all, which feeds her already enormous vanity. Unfortunately, the necklace, like her vanity, has no substance.