Nowhere in the megilla does it say that Vashti was killed. Some suggest that she was taken by Haman (who desired to be king) as his wife. He then changed her name to zeresh. The letters of zeresh are just one off from vashti. That’s how Zereresh knew to tell Haman that if this mordechai was m’zera yehudim, you will fall to bottom. How did she know? what experience did she have? Now if she were of royal decent, it makes sense that she knew. Zeresh/Vashti knew what happened to her grandfather Balshetzer and was tharefore able to predict what would happen to hamman.
And gthere’s alot more…
See Rabbi Aryeh Kaplan