I remember an interesting summary posted on HN back in 2013 about nicotine and tobacco. This was before vaping became popular, but the case he made was that nicotine is actually not very addictive on it own, but seems to reinforce addiction in the presence of other compounds found in tobacco (e.g., MAOIs). Lots of primary sources given.

So what else is in vaping liquid?

"Nicotine as a useful stimulant" (gwern.net)

https://news.ycombinator.com/item?id=6457772