Use of Ecig is banned. Possession is not. And not that I would suggest this but the bathrooms smoke detectors don't pick up vapor. Also you can hold the vapor long enough it will not be visible when you exhale. I would never do it at my seat to many passengers over react to small things.

On a side note somewhat on topic as an Ecig user how in the world has the FDA not started studies on this. Everyone is banning because they are "unhealthy" but no one wants to actually prove or disprove it.