My father has COPD from over 50 years of smoking. I convinced him to switch to vaping and he's been using a Juul for a few years now. Awhile after the switch his primary care physician listened to his lungs and said they sounded a lot better. She said she because the science isn't in "I can't tell you to vape but I can tell you to keep doing what you're doing". Should non-smokers pick up vaping? Probably not. But in…
Nicotine salt is much easier to vape and feels less harsh. I.e I can easily vape 50mg/ml nicotine salt, while above 10mg/ml freebase gets uncomfortable for my throat. That might have been the reason for your dad coughing.
But I still agree, besides the salt/freebase aspect there can be different aspects of the ingrefients, each affecting health differently.