Fired might be different, but if you leave some places won't pay out vacation or only 50%. Some places have learned not paying out banked leave leads to poor outcomes but others still get away with it. Not paying out vacation is a good way to get someone to take all their vacation right before they leave so you get no time to offload work.

Also depends on the state (for example, in New York State employers are mandated to pay out accrued vacation time unless it's a stated, formalized policy not to).