I vaped in the bathroom on our flight back from Puerto Vallarta yesterday, no problem. The bathroom smelled so bad that I figured I was doing people a favor by leaving a little strawberries and cream scent! It is definitely true that too much water vapor can set off a smoke detector (they sense particulate matter in the air, not just smoke), but if you double inhale and then exhale through your shirt or something, there will not be anywhere near enough vapor in the air to set one off. I wouldn't exhale directly at the smoke detector, though!