Earned vacation is a hedge against an inadequate supply of value from the employee. Many states have specific requirements for the payout of vacation, and if you start someone who turns out to be a deadbeat (or someone who leaves within a few weeks for other reasons, like a competing offer came in) with 4 weeks of vacation, you may end up having to pay him an extra 4 weeks of pay for nothing when you fire him, on top of the time you kept him onboard.