Either Electronic Cigarette, or Personal Vaporizer (or derivatives thereof).
The one thing I refuse to call it (& I wish others wouldn't too as it seems to be the will of one person that we should all do it) is E-NI
A. Because the abbreviation sounds silly
&
B. Because Electronic Nicotine Inhalator sounds like a medicinal device, which clearly it is not & the powers that be would be down on us like a ton of bricks if it was marketed as such.
(quite like the sound of Electronic Nicotine Vaporizer though)