A vehicle HVAC system may include a housing, a blower disposed in the housing to blow air, an evaporator core disposed in the housing to cool the air having passed through the blower, a heater core disposed in the housing to increase a temperature of the air having passed through the evaporator core, a temperature door assembly including a pair of slidable temperature door plates to control the air having passed through the blower, and a mode door assembly including a slidable mode door plate to control a vent outlet and a floor outlet such that one thereof is closed, or to control air to be discharged through the vent outlet and the floor outlet.