I have never encountered a private company policy that let you bank your vacation & sick time. Personally, I think only the military should be able to do this, since they're always "on." All the paper-pushing bureaucrats can use it or lose it like the rest of us. If any "banking" is to be done, it should be for time only, not cash. This would come in handy if you had a baby, for instance. Or plastic surgery.
There is good reason that a company or government should require vacation use. Somebody should have to cover your job. I heard about this in accounting class: A woman was considered an exemplary employee, she never took vacation, never called in sick, was always there. She then got hit by a car or something and had no choice. Turns out she was embezzling thousands, and had for years.
