I experienced that first hand as well...when I first started vaping I was not feeling the "Ahhhhh" moments I had when I smoked. It was just all jitter and hyper. I made myself nic sic because I couldn't feel the satisfaction that I got from the cigs so I just kept on vaping. I didn't eat, barely slept, and couldn't stop. At least I wasn't smoking...
Then I read about Whole Tobacco Alkaloids and the MAOI's in tobacco that aren't in our ejuice. Check it out.
I was always taught that "nicotine" was both a stimulant, and a depressant, and your body synthesized the nic differently depending on what you needed. Actually, the alkaloids are the "stabilizer" to the nicotine, and they control (or rather keep) the dopamine that you produce from the nicotine from getting dumped like it would if you weren't a smoker.
So as smokers, we're used to a higher level of dopamine in our systems, and when we quit smoking and vape only, there's no MAOI's to control the dopamine dump. The nicotine triggers more production of dopamine, but our bodies get rid of it more effectively now. This is what causes some of the depressive feelings you might get after you quit, too.