Actually, all of the above has been thought of .. and rejected. First off, vitamins can change from the heat of vaporization, and the amount that could be inhaled in vapor is inconsequential. Nonetheless, you can buy "vitamin" liquids from E-Cig. Caffeine? Why? Energy drink? Janty now has a Red Bull flavor (funny that the label said Red Cow when it was shipped). It's more a taste match than an energy lift. Understand how little substance will be carried in each inhalation. The nicotine concentration is at poisonous levels in the liquid, so that we e-smokers can get some lift when we inhale an individual puff.
There's more on this in the Health section of the forum.
Like some have noted, I'd rather drink a cup of coffee and puff my e-cig than try to cram pure caffeine into e-liquid (can you say "heart palpitations" and "emergency room visit"?). And I don't mind swallowing a few vitamin pills.
A real breakthrough might be including drugs designed for inhalation -- drugs that help lung conditions. COPD sufferers might have new hope to get needed nicotine without worsening their condition. This is a loooong way off, however, given the required testing with clinical trials for all drug delivery.
Right now, the worry is that nicotine will be considered a drug needing regulation.