That instrument is meant to measure the amount (percentage) of carbon dioxide (CO2) in your blood gasses. You take a deep "cleansing breath", exhale completely, take a second very deep breath and hold it for a number of seconds then blow into the device for as long and as hard as you can. CO2 is a normal product of respiration so the reading would never be zero.
As there is no combustion involved in vaping we aren't taking in all the extra CO2 that we did when we smoked. Where I work we do this voluntary "healthy workplace" event every year where they take your blood pressure, calculate your BMI, do a non-fasting blood cholesterol test - and if you wish, for smokers test your CO2%. As a 2PAD, 40 year smoker, when I blew that exact same test over a year ago my CO2% reading was 44 or 45%. When I did it again after about 9 months of vaping exclusively (I was an "accidental quitter" btw) the reading was 2%.