If you requested the vacation and it was approved, then tell the boss that it has already been paid for and it includes other people so you can't cancel it because it would cost them too - it cannot be cancelled because you did not pay for cancellation insurance. If you did not request the vacation and get it approved, then you shouldn't be taking the vacation. When overtime is required, you work it. Unless you made it clear at the interview stage that you would not work overtime, you work when they need you. If you don't want this job, perhaps, it is time to find another job, one that meshes with your life better?
Certified Paralegal, with 25+ years' experience & with Employment law experience.