It goes on the books as a liability, which is why a lot of companies are doing ‘unlimited vacation’ now. (And why others don’t want to roll vacation from one year to the next). If they don’t owe you a certain amount of vacation, they don’t have to pay it out when you quit.
But if they do owe it, they do have to pay you.
That should be true, I agree, however it is not. Here is a source outlining the laws in each state.
http://ask.legalsolutions.thomsonreuters.info/cc-nto-vacatio...
For example, in my state, the law is; Unused vacation will not be not considered wages unless an employment agreement, union contract, or company policy provides that vacation will be cashed out on termination of employment, in which case an employee may have a claim for vacation pay if the terms of the agreement, contract, or policy are not followed (Chrin v. Cambridge Hydrodynamics, Inc., 2003 WL 25754809 (N.J. App. Div. Dec. 30, 2003); see New Jersey Department of Labor & Workforce Development: Wage and Hour Compliance FAQs).
I can't really complain about lack of vacations but I've also never been in a situation where I could really just take a sabbatical for a few months.