I had a job with unlimited vacation once and it's such a strange and uncomfortable concept for me. Upon hiring, my boss told me to take as much vacation as I need/like, "but of course, you cannot go off on for example 4 months of vacation". So, apparently, the vacation is not really unlimited (as 4 months is above the limit) - the limit is merely undisclosed. This policy was one of the reasons I didn't stay in that company for long.