I hate to be a negative voice in this thread, but I do not think vaping caffeine is going to have the desired effects. Some things to realize, some of which have been experienced:
1. Caffeine free-base is very bitter. Chew up a caffeine tablet, like NoDoz.
2. Most people who ingest caffeine need about 100 mg or more to get a desired mental effect. Combine this with the low solubility of caffeine in glycerin or PG and it will require a LOT of vaping to get this much caffeine, and it is not going to taste good at all.
3. Vapor is primarily absorbed orally and in the throat and nose, not the lungs. This includes nicotine in the ejuice.
4. Vaping is
not necessarily the best way to take in all of the many compounds we use, like caffeine and vitamins. Drinking a cup of coffee is simply going to work better than vaping 100 mg of caffeine freebase, and with no irritation to the nose and throat.
Nicotine works in quantities
far lower than caffeine, as in about 1/100. Nic in the stomach can make you nauseous and give you the runs. Thus smoking, vaping or oral absorption with snus, where the nic is kept mostly in the mouth, are the preferred ingestion methods. Caffeine in quantities needed is fine in the stomach.
I understand the interest in vaping caffeine, if not other compounds, like vitamins, but considering the tastes, quantities needed for desired effects, and possible damage to attys, I think vaping these things is simply not a very good way to ingest them. Besides, ever heard of tea leaves being smoked for caffeine? Caffeine has been ingested for thousands of years, and it is always oral ingestion.
And I think publically discussing the possible vaping of compounds other than nicotine, and I have seen threads here where this includes pharma drugs like antidepressants, basically signals to the FDA that we are crazy druggies willing to inhale anything at all without any knowledge of safety, dosages or even efficacy. I think this path with vaping caffeine is not only not effective, but could possibly backfire on us, and further support a gov't ban on PVs.
Just my

.