Of course, any employer can issue an office regulation that says you can't eat or drink at your desk. If they can do that, then they can ban ecig use inside even if it's zero nic.
I work for a reasonably benign company, but with a layer of middle management that apprently has nothing better to do than to troll for "violations" of company policy. If they see a behavior that personally offends them, they cry until a new restrictive policy is adopted ...
Yesterday I got a two page email memo listing all of the electronic appliances that are now forbidden in offices and cubicles. Apparently some guy had a nearly complete kitchen in his office, with refrigerator, microwave, convection oven, and espresso machine, all running from one wall outlet, and he was popping the breakers several times a day ...
The memo says that any violators will have their appliances "confiscated", a lawsuit waiting to happen ...
Guess I'll start using my solar charger at work ...

