Well, if they aren't allowed to purchase them, it would stand to reason that they aren't allowed to use them. Personally, I would much rather find out my kid was vaping instead of smoking, IF they are bound and determined to do one or the other. But the ideal scenario, which I agree with, is for them to do neither.
The only thing that should be in our lungs is clean air. That in and of itself is nearly impossible to come by in our society. Most of us in this forum have used vaping to quit smoking cigarettes, which is wonderful. Some members have actually been able to quit both, and that's ideal too. But to have never done either is the best idea.