At my startup I've decided to have a "take what you want when you want it" vacation and sick day policy. I care about results against goals, not how many hours you work or when they are. Abusing the policy in a way that affects morale (e.g., deciding only to never show up in the office and spend a year working from your batcave) would be grounds for a warning/replacement. So far it has worked well, but with only a handful on the team. Not sure how it will scale to a team of 100 or 1,000 or more.