Engine manifold vac => check valve => reservoir => controller { -> to vents }

Either a cracked, broken, disconnected plastic line or broken pod will cause the system to lose vac and default to defrost & side vents.
You can use any std vac hose. Save parts of plastic vac line to make connectors.

Vents have "pods" that are vacuum operated. Engine vac feeds check valve mounted behind Brake Booster, vac line routes under AC / Heater box, then battery into a reservoir [mounted inside rf fender well]. An output line goes back into eng compartment, then through the firewall to vac controller [mounted on left side of glove box cavity].

Remove the rf tire and plastic fender well [top to rear] to access the dual chamber reservoir. [Be mindful of proper in / out vac lines].
Remove the glove box to access the vac controller.

If vac controller is getting vac, then check each line feeding the vent pods to find the inoperative pod. Remove radio to access vac pod for footwell vents.