> The CDC's website says there's been a 0.6% increase in vaping in middle school and 1.5% increase in high school
No, it doesn't. The 0.6% and 1.5% numbers appear, but as the base incidence in 2011 from which an increase occurred, not the percent increase. Quoting the relevant passage of your own source (emphasis added):
“Nearly 5 of every 100 middle school students (4.9%) reported in 2018 that they used electronic cigarettes in the past 30 days—an increase from 0.6% in 2011.”
“Nearly 21 of every 100 high school students (20.8%) reported in 2018 that they used electronic cigarettes in the past 30 days—an increase from 1.5% in 2011.”
There is a difference between an “an increase FROM x% IN 2011” and “an increase OF x SINCE 2011”; you seem to have confused the former for the latter.
The middle school increase reported is 717%; the high school increase reported is 1287%. There is nothing obviously inconsistent with aggregate 900% increase 2011-2015 with a slight aggregate decline thereafter.