I'm pretty sure that some airlines allow it. I also believe that Ryan Air sell them on the planes too. Someone correct me if I'm wrong.
-edit- looks like the 'e-cig' that Ryan Air sells isn't actually a proper one and it doesn't produce vapour. This is just from googling so I really don't know whether it's all fact or BS.
They are cotton wool like things in cardboard tubes so you are correct they ain't ecigs.
I've stealth vaped on planes loads of times. You double inhale and hold it in for a long time, there is no vapour on exhale. I use 36mg PG only juice and it works fine.