The Book of Esther - more a religious calling to celebrate the Purim feast than a historical document - has proved difficult to place in a historical context. 
"Ahasuerus" should be Xerxes, but the career of his queen Vashti in no way corresponds to that of Xerxes's wife Amestris, nor to any woman of his court known in Persian sources. Josephus identified Ahasuerus as his son , Artaxerxes, and this seems more likely. Among his 4 known concubines or queens, two of them are the 78th great-grandmothers of my children.  One of them was a Babylonian Jewess, who is no doubt the one named Esther in Jewish tradition and the mother of king Darius while in Persian sources is called Kosmartydene. It can't be proven, but would be interesting if the Vashti of the Old Testament was then actually our 78th great-grandmother, Andia, mother of Darius II's queen Parysatis, ancestress of the later Achemenids.