I agree with most of the posters. If the vacation time had been approved in advance, I'd tell my manager what it would cost me out of pocket to cancel my plans, and give him or her the choice of making me whole or letting me take the vacation as scheduled.

I don't do this when the vacation time hasn't been approved yet--in such a case I was taking the risk. I also don't do this when the loss is minimal (say, a $100 change fee) AND I will end up getting at least as much personal benefit out of the trip (like an extra day to play in a city I wouldn't ordinarily get to).

I often run close to my maximum vacation accrual, and schedule a vacation just in time to avoid having the accrual stop. If I were asked to postpone a vacation timed so that that would cause vacation accrual to stop, I would also insist on having an exception made to the maximum, either openly or under the table. At 4 weeks PTO/year, having vacation accrual stop amounts to roughly an 8% pay cut, which I'm certainly not willing to take as a favor to my employer.