I don't know. I know we use them in post surgical patients to prevent pneumonia. By forcing the alveoli to stay open, they are less likely to have collapsed lung which leads to increased incidence of post-operative infection. Perhaps TSM or shakeybrainsurgeon can give you input on this.
It seems to be just too simplistic to improve a healthy person's vital capacity, but I imagine that it can't hurt.
Here is something:
[Effects of pulmonary rehabilitation on vital capacity in patients with chronic pulmonary emphysema][Article in Japanese]
Inoue M, Ohtsu I, Tomioka S, Hagiya M, Sumi M, Aoki H, Homma T, Hasegawa S.
Division of Respiratory Disease, University of Tsukuba, Ibaraki, Japan.
To evaluate the effects of pulmonary rehabilitation on pulmonary function, 15 patients with chronic pulmonary emphysema underwent pulmonary rehabilitation for six weeks as inpatients. Pulmonary rehabilitation consisted of relaxation techniques, breathing retraining, thoracic massage, physical exercise, and walking. In 8 of the 15 patients vital capacity increased by more than 200 ml (over 10%), and in 7 of the 15 patients the load of maximal exercise increased by more than 5 watts (over 10%). Increases in vital capacity were not associated with increases in maximal exercise load. The percent change in vital capacity associated with pulmonary rehabilitation correlated significantly with the percent change in tidal volume and the percent change in expiratory minute ventilation at the maximal load. The percent change in tidal volume at the maximal load correlated significantly with the percent change in maximum oxygen uptake. We attribute the increase in vital capacity to an improvement in thoracic cage movement. These findings suggest that pulmonary rehabilitation can increase vital capacity in some patients with chronic pulmonary emphysema, and that such an increase is not directly connected to increases in exercise capacity.