Really nothing; you're correct to think of it in the same category as caffeine. Both addictive, yet safe.

But vaporizers have more than nicotine. There's other chemicals needed to produce vaporizer fluid, and it's far too soon to tell if they are safe. Even though most of them have been in the food supply, they haven't been inhaled at scale. We already found one that wasn't safe (Vitamin E Acetate), because it was causing very obvious problems. What about the non-obvious problems?

A more direct comparison to caffeine is a nicotine (not tobacco) pouch.