Bioavailability according to one study, when inhaled by experienced smokers, is around 60%. I honestly thought I'd find more research on this, because getting people to inhale caffeine in the name of science sounds fun.

Bioavailability when orally administered is around 99% according to Wikipedia

It's more than next to nil but it's a whole lot less than the standard method of ingesting caffeine.

Problem with inhaling caffeine, isn't bioavailability, it's the sheer amount needed to have an effect.

This stuff is .08%, or .8mg/ml, you'd need to vape 100ml to get the same amount of caffeine as a can of Red Bull (bioavailability aside, vaping 100ml would lead to 80mg caffeine vaped, bioavailability taken into account, it'd be a whole lot more than 100ml needed). On top of that, nicotine is also a stimulant, any subjective effects may in fact be placebo and nicotine.

Second biggest problem is that caffeine is ridiculously bitter, increasing the caffeine content of the juice to something more reasonable, like 10%, or 100mg/ml, it'd be so bitter that it'd make dry hits taste like Nana Cream. It would be a disgustingly painful vape. Caffeine pills don't contain any extra bitterants, chewing one up is pretty much what caffeine tastes like. Yuck. It's horrible.

So, in a nutshell, either someone would have to spend all day vaping at hundreds of watts trying to catch a caffeine buzz on this stuff (and in the end get sick from nicotine or perceive a stimulant effect from nicotine and attribute it to caffeine), or they could try making a juice with more caffeine, then it'd be a matter of not trying to vomit after every hit (and still needing to vape at least a couple ml of the stuff).