Well....I live in a town whose nickname is 'AK-Rowdy'....
We are well known for our (MODERATED), so yeah....I get pulled over on a fairly regular basis.
Though, when I smoked, I got pulled over on a fairly regular basis too..... One of my best friends lives in a SUPER SHADY part of town that also happens to be predominately black/hispanic. Cops generally assume a 20-something white girl only goes to that area for drugs.
I just explain that it's an e-cigarette, show him/her a bottle of branded eliquid that I keep in the car for such purposes (it has a lovely warning label about nicotine being a poison), and they let me go on my merry way. I had once instance where they searched my car, which I found hilarious because I was moving boxes of books over to my mom's, so I had a trunk+backseat+passenger seat full of boxes. They ended up calling back up to search all of the boxes. I sat on the curb giggling like an idiot the entire time.
I've found that dropping my ecig into my lap is pretty much my best option when I see a cop in this town.