It was unheard of for anyone to say no to a king's order and live. Perhaps Vashti hated being treated like a piece of meat. Perhaps, being the queen and mother of Xerxes' heir, she felt that she should receive more honor and respect. For whatever reason, she stood up to her husband, the king, and said NO.
Xerxes consulted with the wise men on what to do with her. He must have loved her somewhat, because she would not be put to death for this crime. But he had to do something. If he just let it pass, then the other women of the kingdom might rise up against men instead of being obedient in everything. Vashti's action could have started a women's liberation movement! Xerxes decided to banish her from the kingdom, and take her title as queen, and give it to another.
We don't know for sure what happened to Vashti after her banishment. Was she discreetly executed? Did she live out the rest of her life in comfort, with family, or in shame? I like to think that perhaps she found happiness somewhere, where she could be valued as a human being, instead of as a plaything. I believe this would be a fitting reward for her refusal to compromise her dignity.