An adult should be able to do whatever the fuck they want, as long as it doesn't impact other people. Vaping doesn't emit any carcinogens or toxic substances, and 10 times less nicotine than smoking does. At the end of the day, vaping does far less harm than smoking, and it's easier to reduce the amount of nicotine consumed with vaping. Nicotine also has health benefits, such as slowing down the onset of Parkinson's.
If teenagers are vaping then that's an enforcement issue, but at the same time I would be less worried if I found a vape in my kid's bedroom than a packet of cigarettes. Teenagers will experiement with substances. Nicotne vapes are way down the list of ones I would be worried about.
Smoking kills 8 million people worldwide every year. I think it's worth pushing the alternatives.