Man,
I don't want to try to tell anyone what to do. Use your own judgement. IMO, for me, I'll drink the caffeine and vape clean juice. Caffeine is more for ingesting than inhaling IMO. I can't think of a good reason to inhale it rather than drink it or take a pill or whatever. Maybe it would be easier on your digestive system to inhale it rather than ingest it. But.... I'd use caution. I don't know the effects of inhaling it. Does the caffeine powder have a carrier of some kind?
If I get the caffeine powder, I'll mix it properly with a liquid to the correct dosage so it's safe to drink. Caffeine can kill ya in high enough doses too. So be careful. That powder is concentrated.
FYI -
AeroShot Inhalable Caffeine Gets FDA Warning Letter | Healthland | TIME.com
This stuff comes up every once in a while. Basically, your lungs are NOT exactly the same as your digestive tract. So, things are different. Just because you can eat it, doesn't mean you can vape it. How hard is it to take a ginseng pill, or drink a caffeinated beverage?
Also, check the ECF DIY section for more info on what is safe in e-juice (like avoiding oils in what you vape).
2 cents.