This won't really work... and here's why.
To even begin to feel the effects of caffeine, you need a dose of around 40mg.. this can me MUCH more for a regular caffeine user (coffee drinker) because of tolerance.
The amount of caffeine in, let's say, a rockstar.... that's around 200-240mg. A Caffeine pill? 200mg.. cup of coffee 120mg you get the picture.
So, let's say that our e-cigs deliver caffeine the same way as nicotine... let's assume (because we don't know any better) that the way our bodies 'pick up' (absorb) and 'use' the caffeine is the exact same as the nicotine (same particle size, same carrier material, both water soluable)
Essentially, if an e-cig was able to deliver the amount of caffeine to actually even feel it, we'd all be dead of nicotine poisoning right now... you'd need an insane amount of caffeine / ml, being that the active dose of caffeine starts at around 40mg... remember, nicotine's active dose is in the *micrograms* (the actual amount absorbed by your body and used, not the amount in the medium) an analogue cigarette only delivers at MOST 1.5mg of nicotine to the body, and we can all agree that analogues deliver nicotine much more efficiently than a PV.. so we can infer that actual nicotine delivery via a PV is relatively low, even with high mg juices.
The same thing will hold true for caffeine, and a 1-3mg dose of caffeine isn't going to do jack squat (and looking at the amount of caffeine used in e-liquid from that link above.. it's actually an extremely small amount to begin with)