Pure nicotine seems to be harmless except in overdose, the main side effect being a compulsive need to consume more nicotine. Whether the delivery mechanism in vaping, year after year, is harmless, is a different question. Anecdotally, I used a nicotine vape to quit smoking -- and they're wonderful for that -- but when I quit that as well, my chest went through a phase where it cleared out all manner of gunk not unlike when I quit smoking cigarettes.