You could allocate those days using "Unpaid" time off type, to "reserve" them. When the time arrives, if the employee has enough balance in Vacations, you can refuse the unpaid leave and request the days as vacations.

I would suggest you allocate the days at a set time of the year and deny any vacation requests if they don't comply with the timing.

The best accural type for this would be to allocate each month a fraction of the days so they could ask for a day and keep asking for the rest of the days until they reach the set time.