The Bible does not teach us what her ultimate fate was, but Esther 1:19-21 seems to describe the punishment fully. Probably she would have remained a “captive” within the palace with all of her “royal privileges” being revoked, probably only her legal privileges as a wife being retained. The fact that she was no longer permitted in the king’s presence explains Esther 2:1. However, she certainly would not have been freed from her marriage to the king, as that might result in paternity problems for him. Also, if the king had children by her already, killing her might not be viewed positively!
We can conjecture that the way that David treated these concubines (they were “put in ward” — separated from both the king and the public but still treated as the king’s concubines) is similar to the fate that Vashti suffered. In David’s case, it was important to make certain that the concubines did not have any additional children, as the parentage would be uncertain (and therefore the succession to the throne). However, they still retained the status (if not the duties) of royal concubines, and Vashti probably retained the status of wife of the king (one of many – different from the Queen) purely to ensure that no one else would dare to mate with her.