Following up: Vacation time isn't a required benefit (in the US), so as long as your employer doesn't prevent you from taking accrued vacation time without compensation before it expires, they can otherwise control when you take vacation. An employer that prevents an employee from using expiring PTO must either compensate them for it or they violate state and federal DOL rules regarding (contractually guaranteed) benefits. Generally the punishment is that the have to compensate the employee, any legal counsel, and pay a rather hefty fine to the appropriate DOL, so it's never worth it to prevent an employee from using expiring PTO. (And if the employee is so valuable that the employer can't afford for them to take PTO, that should have been negotiated into the contract.)

However, if the employee "chooses" not to use up all their accrued vacation time before it expires, generally compensation is not required...