I know that this seems harsh but under most state's laws it is legal. The majority of employment arrangements are wht is referred to as "at will". This means that an employer can hire or fire someone for any reason or no reason at all. It can also increase/decrease salary/hours, promote/demote, and generally impose requirements as it sees fit. So absent a prohibition against your employer's action via a union agreement, an employment agreement, stated company policy or actionable discrimination, no law is being violated.
Further, while employees generally see vacation as an automatic employment that can use as they see fit, the law doesn't take the same view. The fact is that vacations are not legally required. Therefore, to the extent that an employer institutes a vacation policy it is a discretionary benefit and as such an employer can design it any way it chooses. This includes when and whether to mandate that such time be taken, with or without an employee's knowledge or consent.