A system and method for controlling a combined heating and cooling vapor compression system are provided. The apparatus may be a vehicle and may include a cabin, a vehicle battery, a Rechargeable Energy Storage System (RESS), and a vapor-compression system, having at least one controller, operable in a variety of modes selectable to facilitate cooling, heating, and dehumidification of the vehicle cabin. The method may include steps to adjust evaporator air temperature to control comfort, fogging and smell in the cabin by adjusting the compressor speed; regulate heat pump performance by adjusting the output of an electric heater and adjusting the flow of coolant through the RESS chiller; and evaluating and optimizing the discharge pressure and suction pressure of the compressor by adjusting the compressor speed and adjusting the coolant flow through the RESS chiller.