I used to fly as a cabin crew and used my pv in the toilet as well as the crew bunk. It will not activate the smoke alarm in the toilet, but still gotta vape discreetly. No doubt its just vapour, the sight of it will alarm the rest of the passengers and may get them thinking that the smoke came from the plane itself.
As for customs, the small amount of
juice is not illegal to bring into. However, it still depends of whether the officer might make a big woo ha and confiscate it based on their own ignorance.