"Even if there are long-term vapers, this is not a problem, as long as they quit smoking. The problem is combusted tobacco, not nicotine. At the dosage used by vapers or users of nicotine gums or patches, nicotine is not toxic. Long term vaping is not a public health problem; not any more than long term use of nicotine gums."

interview is here:

Interview with Professor Jean-Francois Etter on Long-Term Vaping

also below is a bit of a write up of the tests he and Dr. Chris Bullen (Clinical Trials Research Unit, University of Auckland) conducted:

New Study Looks at the Long-Term Behavior of Vapers and Dual Users