It's been discussed here:
Smoke Alarms &
Do e-cigs set off smoke detectors?
I don't blame you for not seeing them. It's several pages back.
To save you the time of reading through them, the conclusion was that it depends on how sensitive the alarms/detectors are, and what brand/type they are. Although various people have mentioned that they've managed to vape in bathrooms
undetected. Whether or not it's worth it, is up to the individual.
Personally, I think it's kind of gross. I mean, we're talking about really tiny bathrooms, unless your fortunate enough to be in the business or first-class section of the plane it's still quite a confined area.
Plus you've got to deal with all the sex juice(Mile High Clubbers) & poopie fumes that are flowing in there. Okay, maybe I'm being a little paranoid. Still I try my best to get in and get out a.s.a.p when it comes to airplane/aircraft latrines.