The entire idea is that vaping replaces the cigarette and most of the harmful substances in the tobacco. Cigarettes don't provide a kick because it's full of the shit that makes your lungs go black. It's the nicotine. And while nicotine is no saint, it's not the main culprit of the health problems smokers face.

Remove cigs, replace them with vapes. Presumably the effect is the same, but the vapers live slightly longer than they would've if they were smokers.

I feel like increased productivity from smoking tobacco wouldn't be impossible to calculate if you just compare performance of smoker vs non-smoker in aggregate.

I would be surprised if the stimulating effects outstripped the dying early effects.

edit: just did a brief search and apparently on aggregate former smokers who quit have increased productivity following quitting, so even while smoking there is decreased productivity effects