Companies vary greatly regarding willingness to grant unpaid vacation time on top of the vacation provided. The reason against unpaid vacation is there is work to be done, and theoretically if everyone took some unpaid vacation they'd need to hire more workers, and the cost of hiring and training each worker is significant.

Unpaid vacation, in my experience, is most often given when a job seeker starts a new position and has travel plans in the first few months. If the hire hasn't accrued their vacation time (and doesn't want to burn it all up right away), an unpaid leave is usually a good option.