Show evidence of notable addiction to nicotine in humans without other tobacco smoke compounds involved (as they aren't involved in vaping). You know, the sort of evidence where there's a measurable behavioural response.
No, really. There just isn't any - partially due to their being no appetite for researching it, and partially because until recently, getting nicotine recreationally without smoke being involved (bar Snus) was basically just not on the radar.
Fact remains that the only time you see evidence of anything approaching addiction in relation to nicotine is when tobacco smoke is involved.
The very fact that the vast majority of users of these devices start off at 24/18/12mg nicotine, and tend to scale down to 3/6mg by choice without any problems should suggest that nicotine, in and of itself, just isn't all that addictive. Otherwise, it'd go the other way - people would start at 12/18 and go up to 24/36. The act of vaping itself may be 'addictive' or habitual (the feeling of vapour filling the lungs etc can be like biting nails or cracking knuckles) but the nicotine component of the habit is, charitably, questionable.
Addiction is a complex, often pejorative term; and smoking was never considered an addiction until the late 80s, and it was based on some very dubious evidence:
Fact is, eve if we do say it's addictive - addiction isn't a problem if it doesn't negatively impact the user or their peers. It's just a self-reinforcing habit. And we have little evidence to support even that, when tobacco smoke isn't involved.