A/c works but it blows out on the windshield and floor but not the vents

you more than likely have a vacuum leak under the hood where the small plastic vacuum lines are at the firewall coming into the cabin.as these small hoses age they crack/split/dry rot and you lose vacuum.when this happens the air direction usually defaults to the defrost mode no matter where you set the dial. this is a cheap fix,you can buy the fittings/hose at local parts stores to repair your damaged hose(s).sometimes you can hear the leak by a hissing sound,engine running,A/C on and/or just run your hand/fingers around the hoses to feel for a split/dry rotted hose/line.CAREFUL when doing this,engine is running/hot,fan will be spinning!