The employee spent the time at the firm that it took for the vest to happen. That time is mostly already served. Yanking the promised reward like that does not send a good signal to the rest of the staff. If the employee was so bad they should have fired them long ago, for the sake of the rest of the team if that was a concern. But a month ahead of the vesting? That is a shitty move. In this scenario, the eventual firing is sufficient signal to the rest of the team that the person was not living up to their paycheck.