My parents found mine as well, but they took it differently. I hid smoking from my parents for about 5 years. And came clean about it last year, when I was 20. My parents are happy about the fact I'm trying to quit, as yours should be as well. They just don't allow me to vape around them or in the house (which I do in my room anyways). There's really not much you can say to your parents, you said it all already. You're a grown adult, and you can make your own decisions on what you do and don't do. And you're obviously making a great decision by switching over to the ecig.
