When strangers (specifically ones who might ask or tell me not to use it where I am) ask me what mine is, I have started saying "What, this? Oh, it's really just a nicotine inhaler, but it also makes little puffs of steam so it's more effective than the old-fashioned ones." Which is true, and although I haven't made any specific claims about it being some sort of NRT or stop-smoking-aid they immediately see me as someone trying to give up smoking (and thus should be encouraged and supported) instead of an unrepentant smoker exploiting a loophole to get around a smoking-ban.
The very first words you use will quickly cement in their minds which way to view e-cigs and their users. I'd probably lead of with something like "You might have heard about these things, or seen them on TV. They're called electronic cigarettes, or [X] or [Y]. If you've never seen one in action, here it is. [puff puff] If you're like me when I first saw it, you're wondering what the hell is going on here? Well, I like to think of it as [what I said above]. I bought one [X months ago] and I haven't smoked any real cigarettes since then." Right there, you haven't made any specific legal or health claims. You've used simple, everyday, personable language to relate an anecdote so that their first-impression of e-cigs and their users is a favorable one. Everyone likes a successful kicking-addiction-story, so they're going to be favorably presdisposed to whatever you say next.
As for the drug-paraphenalia angle....
"The atomizer with its small wire heating element is basically a slightly more robust version of a common lightbulb--and burns out almost as easily. You can't simply throw some tobacco in it--it wouldn't work, and would probably cause the device to burn itself out. Similarly, a person simply can't put marijuana, ......., or any other street drug into it. They not only wouldn't get high, they'd ruin both their drugs and their e-cig."
