Nearly everything in this Minimum Vacation Policy post from Travis-CI is spot on. I've never been a supporter or believer of the unlimited vacation policy. Having managed teams for a long time I know that the majority of people faced with uncertain guidance like "unlimited" just will never take it. The dialogue inside someones head is something like:
Is it really unlimited? Of course not, there is some limit. What is that limit? I don't know. Well, I better not take any time off.
Even worse I think this type of policy is a punt on the part of the leadership team to not actually specify what the operating practices for the organization are. Oftentimes this is considered "cool" but I think it’s really just shortsighted.
The idea of instituting required vacation each year however is interesting and is probably a very healthy thing. I think you can get much of the same affect by having the leaders in the organization set a good example of taking time away and disconnecting.