Interesting. You're all far more reserved than I am with regard to my exposing my children to reality. The reality is, drugs suck. Don't even start. They saw my wife struggle with quitting and they're now watching me. Unromanticized, unhidden -- the realities of being a slave to a substance and the realities of trying to quit. I can't think of a better example.
If your children have no frame of reference, what is to prevent them from making the wrong choice when they're presented with the option? In other words, you sneaking off out-of-sight into the backyard promotes curiosity and prevents an honest conversation about substance abuse; be it niccotine, alcohol, Marijuana, ......, whatever.
I'm taking this eCig route in an effort to quit. I don't want psycho-active drugs (Chantix). If my children see that I've made an admittedly bad mistake and am trying to correct that mistake, I think that speaks volumes about making bad decisions and trying to rectify them. Adults make mistakes. Isn't that the wisdom we, as parents, impart to our children..? We make mistakes and we correct those mistakes. Don't make those same mistakes -- make your own and teach your children of those mistakes so they're not repeated.
The theory being my children will not, when offered a cigarette, say, "...oh, these are the things Dad used to sneak into the backyard to do! They're so good he doesn't want to share them with anyone else and hides it from us.. I wonder why they're so good..? <Puff>" No, my children will hopefully say, "...these are the things Dad sweated over for 4 weeks trying to quit. They made him spit up black stuff, they made his lungs feel like sticky bricks when he tried to breathe deep... no thanks!"
I hide nothing from my children, not my triumphs or mistakes. I don't want them trying to live up to some unrealistic expectation of perfection in mind, body and spirit. It doesn't exist anywhere. I'm a human, like them, subject to bad judgement -- the WISDOM comes from recognizing your mistakes and being big enough to admit to them and correct them.
As for vaping -- I think the carcinogens given off during cooking anything would probably contain more parts-per-million than glycerin vapor -- same with home heating and the biproducts of combustion blowing through the forced air vents.