There seems to be a vocal battle between e-cig users and scientists each having their own claims about diacetyl. It's the famous fake butter compound for microwave popcorn that caused lung cancer in the workers who handled it.
My dad has IPF and COPD two lung diseases from years of working a blue collar job breathing in small damaging particles over time. Sawdust, paint drops, metal from grinding, probably welding fumes, etc. any small particles may seem harmless but over time damage lungs.
What concerns me about e-cigarettes is maybe the damage doesn't appear until 30 from now. Will we see an increase of lung disease 30 years from now?