My two concerns are that, you wouldn't hold the vapor in your mouth long enough for it to actually be helpful, and with some people vaping occasionally, and others doing it constantly, it would be difficult to keep the fluoride levels high enough for casual users to get a benefit, but low enough that all-day every-day folks don't OD.
Why make them illegal? I used a vape to limit my nicotine intake and weaned myself off of smoking (and vaping) completely. It still sucked for the first 6 months and the occasional random craving that comes from being a smoker for over a decade, but at least I fought each part of the smoking addictions separately which made it bearable.
For clarity, I got 0% nicotine vape juice for a month or so before I quit puffing completely so let the withdrawal from the substance run its course, and then had to deal with the psychological side of things. Which was VERY annoying. Everytime I'd do anything where I'd normally smoke triggered a craving.