Back when I used one of the first generation e-cigarettes, and the rules were not clear, I asked a member of (VS) crew whether I could use it in the bar area / vestibule.

The crew member said no; but the reason given was that “we know they won’t cause a fire or pose any real danger, but if people see you with them it might encourage (actual) smokers to light up in the lavs”

only one data point but I always found that reason to make some sort of sense