Why would a smoker may have a lower vital capacity?

Structural damage: To the lungs. Smoking usually doesn't have effects of the vital capacity, but rather of the ability of the lungs to be elastic and push air out rapdily (forced vital capacity and expiratory volume). However, vital capacity can be affected by itself from smoking related damage. If you have a lower vc, your lungs are being damaged. Stop smoking, get help if you need it. Talk to doctor who did test.