My guess is that it comes down to active dosages, and that it would be impossible to
vape caffeine in any noticeable quantity.
Caffeine from a cup of coffee is about 80-120mg.
Nicotine from a cigarette is about 1mg.
The concentrations of caffeine that are even soluble are probably quite low, meaning you have to
vape an enormous amount to get your caffeine fix.
I can't fault anyone for laziness and novelty, though. I take green tea pills and the occasional Penguin mints.