A few steps in careful analysis of the Biblical books of Esther and Daniel produce a compelling case for Vashti having lived in the period 620-539BCE. The key is the status and role of her husband Ahasuerus. In the book of Esther Ahasuerus is the husband of Vashti and Esther. In the book of Daniel he is the father of Darius the Mede, who was aged sixty-two in 539 BCE. So Darius’ father was born around 620 BCE. But are they the same person?
It is very likely that the father of Darius the Mede was Astyages the son of King Cyaxares of Media who reigned from 625-585BCE. Ahasuerus (derived from ‘Achashverosh’, a signal term used by Cyaxares in the defeat of the Scythians who initially usurped his throne) was a generic name for Median kings. As such it cannot be confused, (no matter how many linguists or other hopefuls fiddle with it) with the Persian King Xerxes, who was born around 100 years after Astyages (generically titled Ahasuerus) who ruled from 585 to 550 BCE.
The beginning of the book of Esther tells us that Ahasuerus ruled 127 satraps of the Medes and the Persians from India to Ethiopia from Susa, the former capital of Elam. He was not a Persian king, as the then Persian king was Cyrus I of Anshan, situated further south east of Susa. So a Median king named Ahasuerus, not a Persian king, ruled the 127 satraps and was the husband of Vashti and Esther.
The Ahasuerus in Ezra 4:6 apparently lived after Cyrus King of Persia, (Cyrus the Great) who ruled from 559-539BCE. He is not Darius the Mede who died around 537 BCE. No more Median kings are mentioned in history or the bible. This Ahasuerus is correctly not denoted as the husband of Vashti and Esther and is probably Xerxes I (it is still not clear how the Hebrew Ahasuerus can become Xerxes) who has never been termed a Median although he had Median blood.
We can only conclude from biblical evidence and accepted historical facts that the father of Darius the Mede was the husband of Vashti and Esther, placing the events of the book of Esther in the period of 585-575 BCE.