Ok, so you can reason this by drawing out REIT (sorry I can't draw it here). Anywho, you know its a restrictive disorder and will have a decreased RV, so automatically you know that RV is a component of FRC (FRC= RV+ERV). Thus, FRC should also be decreased. You also know that restrictive diseases are characterized by a steady decrease in FEV1/FVC since both components are decreasing. This leaves you with VC decreasing since FVC= forced vital capacity.