I automatically pick it up and hold it screen away thumb on button, but it's a lot more comfortable the other way, so when I remember, I use my index finger on the button. If I remember to pick it up that way to begin with, there's less fiddling to get to holding it in a way that's ready to fire.
It seems made for finger rather than thumb.
As long as we're on thumb vs. finger, which hand do you vape with, and are you right or left handed? For example, I'm left handed, but seem to always vape with my right hand, thumb on button.