Manifold vacuum connects through a hose to a port on one side of the vacuum check valve. The port on the opposite side of the check valve tees into two hoses. One hose connects to the vacuum reservoir (the black plastic ball mounted on the firewall) and the second hose runs through the firewall inside the cab where it connects to a vacuum selector valve that is integral with the HVAC control head mounted in the dash (the panel with the A/C, heater, and fan controls). The reason defrost air doesn't switch from the floor nozzles and blow onto the windshield is that the system depends on a continuous supply of vacuum to power pneumatic actuator motors that selectively move air doors within the HVAC housing behind the dash to direct airflow out the various nozzles, depending on what is selected.
So, the solution is fairly simple. Replace that broken check valve.