That might be against the law, the state I'm from has law that says “no contract or agreement can require the forfeiture of an earned benefit” - like vacation.
In Arizona, an employer doesn’t have to pay out vacation hours when an employee is terminated. While it may be legal, in my opinion it’s a disgusting thing to do to a longtime employee. He was in sales, unfortunately, and although he met or exceeded his quota for 18 consecutive quarters, the last 2 he did not, so they threw him out. Buncha a-holes.