FYI,
Vapor WILL set off smoke alarms on planes...See an old Post by Cash (Puresmoker).
Best - Mr.e
Im sorry, not necessarily true (no disrespect to you or Cash).
There are many types of smoke detectors. The most common working principles are photoelectric and ionic. The first one sees the smoke; the particles get into a chamber and obscure the path of light. The later smells the smoke; it contains a radioactive ionizing material and when the particles enter the chamber, the electrons removed from the air cannot continue the electric current creating an alarm.
More likely a photoelectric detector wont be triggered by vapor but an Ionic could. There is no way to know the type seeing the casing. As far as I know, old planes have photoelectric detectors but newer ones could even have early smoke detection (which certainly will detect vapor) or the latest particle detection technology which wont be triggered by vapor.
Now, some smoke detectors have a thermal component, and can sense raise or change in temperature; if the detectors have a really small delta, then certainly they could be triggered by vapor.
To be safe, I prefer to stealth at my seat