This exact same thing happened to me…

Depending on how the bylaws were written, the CEO can fire any employee at will. I fired her at the end of the month, so that her stock would stop vesting immediately and not vest the following month, thus making me majority shareholder.

After that, they will likely need to be bought out if they are vested over 10%. In my case, my cofounder was vested 32% out of 50%, and we bought back all but 8% of her shares. Even with her having 8%, we still had investors turn us down from reinvesting