The best way to find out for sure and get the official policy for any airline is to ask them if they allow e-cigarettes. Some airlines has issued their position on e-cigarettes. Some airplanes permit e-cigarettes. Some do not.
Getting the answer becomes more complicated when you deal with international airlines.
The same holes turn for air ports and where you can smoke ecigarettes prior to boarding airplanes.
What I can tell you is that from my own personal opinion is that you can smoke at the airport and on airplanes discretely without any one knowing or causing any problems. I usually do this whereever I am.
If I am at the airport or on the airplane, I just go into the bathroom and take out my e-cigarette and puff away. It doesn’t emit any odor and the fake smoke that does come out is all water vapor and disappears very quickly.
By the time that finish smoking and leave the bathroom, the smoke is gone and there is no lingering smell so no one even notices. I have also done it while in my seat after everyone goes to sleep. I just hold my hand in a position that hides the e cigarette and smoke it really fast. I don’t recommend this last method if the airplane is really busy.
Just go to the bathroom and you will be fine.