Vashti, too, was actuated by a political motive when she determined to give her banquet. By inviting the wives of hostages in case the men rose in insurrection against the king. For Vashti knew the ways of statecraft. She not only was the wife of a king, but also the daughter of a king, of Belshazzar. The night of Belshazzar's murder in his own palace, Vashti, alarmed by the confusion that ensued, and not knowing of the death of her father, fled to the apartments in which he was in the habit of sitting. The Median Darius had already ascended the throne of Belshazzar, and so it happened that Vashti, instead of finding the hoped-for refuge with her father, ran straight into the hands of his successor. But he had compassion with her, and gave her to his son Ahasuerus for wife.
Belshazzar in Jewish Tradition
Legends of the Jews 12:28