This makes no sense. There's a conflict between the employer (who wants more work), and the employee (who is exhausted and wants some time to rest). What happens if the employee refuses to continue working without a vacation? If the employer backs down then the employee has the bargaining power. If, on the other hand, the employee is swiftly fired and replaced with a more compliant one, then perhaps there isn't as much of a shortage as you claim.
Anyway, this shouldn't even be a conflict to begin with. Exhausted programmers produce crap that keeps breaking, and their sleep-deprived fixes maybe put the worst fires out, if you're lucky.