The longer I'm around, the more I realize that the mechanism of addiction(s) varies from person to person. As others have noted, some folks who were heavy smokers seem quite content at fairly modest nicotine concentrations with their vaping. Others who were what I would consider light smokers seem to require higher nicotine concentrations. There doesn't appear to be too much rhyme or reason to it all.
...
You know, it actually makes sense. Those that smoked lightly got a nic buzz more often, and each cigarette significantly raised their nic blood level. The crave isn't from the nicotine itself, but more for the spike in its content in one's blood.
Heavy smokers are used to a constant nic level in their blood, so a constant vape is all they need, not necessarily a high content.
Thinking about it... I miss that buzz =/