You are operating on the mistaken assumption that the number of hours an exempt employee works has any meaning. It does not.

An exempt employee not paid on the basis of how many hours they work; they are paid on the value of the job to the company. So working over or under 40 hours is meaningless. You have to be paid the exact same amount regardless of the number of hours you work, whether it's 15 or 75, with limited exceptions.

However, the Federal government has gone on record saying that they do not have any interest whatsoever in what happens to vacation balances, and while a state CAN implement additional protections over and above Federal, as yet no state has done so (certain limitations in CA, but not ones that would affect this situation even if you were in CA). It is entirely up to the employer whether or not to charge an exempt employee for a vacation (or sick) day if they do not come in, regardless of how many or how few hours they may have worked at other times in the week.

In fact, in many situations, it would be legal for that last day to be UNPAID, even for an exempt employee (see reference to exceptions, above). So allowing vacation (or sick) time to cover that time is, in itself, a protection.