Clarification Re: UNPAID Vacation.
You do not have to book unpaid vacation.
The company cannot force you to take unpaid vacation time. However, also remember if the vacation allotments are full the company does not have to schedule unpaid vacation time – for example – if you have 40 hours of unpaid vacation time and you want to use it in June and the allotments are full, the company does not have to give you the unpaid time off.