I do understand the first reaction of seeing someone who doesn't have a cigarette addiction take up vaping, especially with nic. We have been told for years that smoking is bad (and for good reason), and now those same people are changing the dialog to include e-cigs. It is a normal human reaction to attempt to keep those we care about from making the same bad choices that we've made, and it's commendable.
However, I'm seeing a trend of potential misinformation in this thread that concerns me.
While nicotine is addictive, the majority of the research done on the addictive qualities of nicotine have been related to cigarette smoking, which is known to include a number of other chemicals that enhance nicotine delivery, and therefore enforce addiction. If nicotine itself was as addictive as some people are saying, then the use of NRT products would likely be equivalent to dispensing natural opioids at a methadone clinic. While more research may needed to determine the real (observed) addictive quality of nicotine, my personal experience seems to indicate that nicotine alone has only a slightly higher addiction rate than caffeine, both of which I consume on a daily basis. Also, please keep in mind that addiction itself varies from one individual to the next, which has been concluded in several studies on chemical dependency and addictive behavior.
As for the gateway argument, really? I don't want to come across as snarky on this point, but it does just seem ridiculous to me. This has been an argument that has been around as long as I can remember with regard to many substances and activities. To this date, I don't know a single person that has gone from a relatively benign substance (not to be mentioned here) to a hard drug user, yet this argument is still used in that context to this day. I suppose that I could be one of the few very lucky people who have never experienced or witnessed this scenario, but given all the fervor and supposed data, I would think that I would know at least one person who fell into this trap.
As to the OP: My opinion is that if it's your son or daughter making the decision to vape, then it's up to you to do what you believe is right as a parent. Not everyone has the same parenting goals or style, and I don't believe that any parent is perfect. We all just do what we must do to make sure our kids are as happy and healthy as possible.