Your argument is that vape using teenagers were unlikely to have smoked. I think it's hard to tell if vaping is diverting teenagers who otherwise would have smoked, or simply creating a new habit.

There are some intertwingled effects where increased vape use (and decreased smoker population) might be contributing to the unfashionability of smoking, both among teenagers and in society at large.

Perhaps we could compare teenage smoking rates in countries where e-cigarettes are banned / unavailable versus countries where they are available. I don't have this data or any studies to point to, but I would be interested to know more.