The one time I tried to do this my employer would no longer approved vacation requests and refused to pay out my 3 weeks of unused vacation.
That is overtly illegal in most/all of the US. Accrued vacation is like money, it is already yours. It's why so many companies have now switched to unlimited vacation, to get that liability off the books. The icing on the cake being that most people end up taking less vacation, not more, when you remove the limit.