I’ve never worked a job where I had to give a reason for using vacation/personal time, or comp time. In fact, I think it would be very unusual for an employer to demand a reason for taking time off that was due an employee.
This isn’t vacation pay. This is on top of paid vacation. This is called paid leave.
I worked at an organization that had generous sick leave and vacation policies. Just because you have time on the books doesn't mean they have to grant you leave on the spot. If I was going to take a lengthy vacation I would notify my bosses several months in advance.