I think "unlimited vacation" should not be conflated with "vacation any time you want it". To avoid the pathological cases you mention above, managers still nee

I think "unlimited vacation" should not be conflated with "vacation any time you want it". To avoid the pathological cases you mention above, managers still need to make sure that not too many vacations overlap. If expectations have been set that you can take vacation whenever you want (even during a crunch on a project that you're leading) without consideration of the business needs, I'd suggest that's a big mistake.