Simply put, a vacation is ignoring the schedule and remaining in unoccupied setback for a day or days.
Vacations are contextual. If you set a site wide building vacation. ALL zones will ignore the schedule for that day.
If you set a vacation for only a specific zone, only that zone will remain in setback for the vacation period you defined.
To learn more about building vacations, check out this article.
To learn more about zone vacations, check out this article.
Setting Zone Level Vacation Schedule from Facilisight