What could scary results from a pulmonary screening test mean? At a recent health fair in my community, I took a pulmonary screening test that showed a "diminished vital capacity". The technician said I should see my doctor. What could this mean?

Depends. Depends on other factors: smoking, copd, asthma could lead to this condition. Tech is not right person for advice. Talk to the doctor.
A . A vital capacity is a measurement of the volume of air exhaled from a fully inflated lung (i.e. From a position after you have inhaled as deep as a breath you can take) to a fully exhaled breath (i.e. Until you can no longer breath any more air out of your lungs). On its own, having a reduced vital capacity measurement just tells you that you have a lower than normal lung capacity. Having a reduced vital capacity is classified into two distinct disorders - either a restrictive defect (e.g. Due to a lung or chest wall disorder) or obstructive disease (e.g. Due to copd). Further measurements apart from just a vital capacity are obtained when performing a full pulmonary function test, and this is needed to identify the exact nature of the problem. It is likely that only a vital capacity was measured at the community health fair, and you should talk to your doctor about obtaining full pulmonary function tests to determine if there is an actual lung disorder and if so, exactly what it is.