I work for a company that's been non-smoking since 1999. If you are on the clock, on the property, at the job site, or just anywhere at all near the company or company equipment, smoking is not allowed. We don't care if it's your lunch break - since no one is to leave the job site even at lunch, that still means no cigarette.
My boss doesn't know I smoke. How he hasn't noticed in almost a year is beyond me, but he hasn't. So he has no reason to know about an e-cig. Since I'm alone in the office 90% of the time, I could and probably will use the e-cig while at work.