Usually its ok if your moderately discreet. I have vaped in many US airports, Milano Linate, Frankfort, Tokyo Narita and Ontario international. All of them allowed it. I did smoke on planes as well, although when someone complained I was asked to step to the back of the plane. DO NOT vape in the bathroom of a plane. Some of the smoke sensors are so sensitive that the water vapor will set them off. If you are worried, wait until you can move around and ask a flight personnel member.