How do we resolve this fine line between vacation leave and FMLA leave?

Q: “We have an employee who has been approved for intermittent FMLA leave to care for his mother who lives in Michigan. (He has provided the medical certification.) In order for our plant to run efficiently, we only allow a limited number of employees to be on vacation at one time. Our vacation year runs from January-December and employees who do not use their vacation days lose them. This particular employee hangs on to his days (at the moment he has 14 left). He wants off the week of Thanksgiving and the days between Christmas and New Year’s, but was denied because other employees have already scheduled vacation for that time. Last year he did the same thing, and then the week before Thanksgiving he told us that he needed to use FMLA for Thanksgiving week. Can we prevent this from occurring again?” – Gayla, Alabama