So it seems like alcohol addiction is not the only substance addiction creating flight safety issues. Yet the world (of aviation culture and of law) is rightly hard on drinking and flying, but smoking and flying gets a pass.
Apart from apparently causing a tendency to try to remove the air supply from the aircraft in flight, what if the e-cig catches fire or explodes? It's not going to do the flight crew or the cockpit any favours to have
the e-cig explode in the user's face and blast fragments around the cockpit at high speed.
(Occasionally gory compilations of exploding e-cigs can be found on youtube, not linked here because the preview is quite grim)