How bad does it mean when spirometry results only show the number 300 ml (vital capacity)? What is the average normal value for spirometry test?

Depends on. The predicted normal lung function results are based on age, sex, ethnicity, gender, and height. Thus the normal value varies among individuals. You must have misread your vital capacity since no adult can live with a 300 ml vital capacity. Could it be 3, 000 ml?
Vital capacity. The formula for vital capacity calculation is the following: vc_{female} = (21.78 - 0.101 a )x h where vc stands for vital capacity, a for age in years, h for height in centimeters. Hopefully, that helps.