I personally don't think caffiene would be safe to inhale. However, I never read any studies thus far. I only vape the Vitamin C option. (Absorbic Acid). It is said to help facilitate healing of delicate lung tissues. I dont know about the other vitamins. I would be afraid of side/adverse effects from them. Of course, its a personal option.
Now if they could put ephedra back on the market and market that in e-liquid I would buy it. It is great for the lungs as well. However, it is a hard find on the internet.