here is the way i see it... and I am not a scientist, so this is just a rationalization.
So smoking, you smoke, get the nicotine, with all the burning plant matter, and crap thats in a cigarette, so while you get a little head rush from a blast of nicotine, some of the nicotine takes longer to get to your blood stream as its in with the tar and other crap that gunks up your black lungs, so some gets through right away, while some doesnt. some is kinda gradual.
We all know that steam rooms are healthy, so breathing steam in to your lungs is not a problem. As you vape, the lungs take less time to process the steam.(As there is no gunk) so the nicotine goes right to your blood stream. this means a faster hit, albeit shorter, getting all the nicotine at once. al though it goes away faster. Chain vaping, Which many do because they like the flavor.... means a lot of nicotine faster, which means more of a hit, but overall less absorbtion.... more absorbtion right away. I think this is why it becomes so easy for people to lower their nic levels. as time goes on, they realize that they are gettign too much at once... etc.