Not clearly defining the benefit is to the advantage of the employer. A defined vacation policy means that if you're fired, the company needs to pay out your unused vacation. It means that if your boss isn't letting you take it, there's a problem between the written policy and your boss's preference, and that might mean a resolution mechanism. It means additional ammunition when asking because it's your due, not a favor. It means that the person who is not socially adroit understands how much vacation they're allowed and isn't leaving available vacation unused.