The best system I've seen is a well-tracked generous allocation of N days per year, with the option to buy or sell 10 days per year. I think this is the fairest

The best system I've seen is a well-tracked generous allocation of N days per year, with the option to buy or sell 10 days per year. I think this is the fairest way of doing things; if you don't care for vacation then you can increase your salary, and if you want more vacation you can pay to get it. In practice the "unlimited vacation" seems hard to scale beyond a small team, though in that case it can be made to work. It relies on trust, which I'm skeptical would scale to hundreds of people. That said while you're small I think teams can get away with it if management lead by example and actually take good chunks of vacation.