I'm going to split a hair here.
It is 100% legal for an employer to require both exempt and non-exempt employees to use vacation time for the days after the holidays if they are not open. They may do so with or without the employee's consent or even prior knowledge. The ONLY exception would be if a legally binding and enforceable contract or CBA expressly and in so many words stated otherwise. I have yet to see a contract or CBA that has such a provision, after 35 years.
It is 100% legal for an employer to require a non-exempt employee who does not have available vacation time for these days to take the time unpaid.
However, it is only legal for the employer to require an exempt employee who does not have available vacation time for these days to take the time unpaid if the employee does NO WORK AT ALL for the entire week. If an exempt employee works at all during those holiday weeks, he must be paid for those Fridays. It can be with vacation dollars - the law does not give a hoot what bucket the time comes out of. But it cannot be unpaid unless the employee does no WORK during the week.
Note that I am not talking about being paid for the holiday. Holiday pay does not work make.