My current boss values vacations--his own, and his employees' also. I work in a school, and even though I do work through the summer, I can't imagine being asked to cancel.
My husband's current employer cannot cancel an employee's vacation after the request has been approved, per the contract.
A former employer of his did ask him to cancel once, the day before we were supposed to leave. It was basically hinted that if he took the time off, he would not have a job to come back to (no union, no contract). This was not surprising, he had suspected it was coming. He had a copy of our vacation rental contract (a house on the Outer Banks) with him, and told his boss he didn't have vacation insurance and couldn't afford to lose the money; if they wanted him to cancel, he would need a check for the full amount of the rental before he left that day. They called him to accounting five minutes before he left to pick up the check. He was furious, but we were not in a place to be without his job at that time. It happened to be the day after Disney released the 7 for 4 deal in 2004, and I called as soon as he got home and booked a trip for three months later. He left that job not long after we took that trip.