There's a name for this, I can't recall it right now. I heard it recently in a discussion about a change in some aspect of wealth distribution in America. This had been changing on a straight line since around 1980. For 8 years it was Bush's fault. Then it became Obama's fault for the last 6 years. Soon someone else will start getting the blame for it.
I'm not sure how you would prove a cause and effect relationship between a decrease in smoking and an increase in
vaping, even if it's there.
SmokeyJoe talks about the 20% Prevalence Rate effect. This applies to the population as a whole, I don't know if 20PR can be applied directly to teenagers. Does a drop in the smoking rate among teens mean there will be a drop among adults down the road, or
just people starting later? Is the anti propaganda working? Are teens smarter these days? Is the availability of vaping reducing the movement of teens into smoking? Is 20PR breaking down? Everyone will tend to apply their own best guess to it. Common sense indicates to me that there would be a connection, but that's not proof. I haven't seen any convincing proof though it may be there and I haven't come across it.