There's a middle ground, here.
Ask first, and be polite.
I always ask first. In restaurants, coffee houses, even on a train. Think of it like not wearing a shirt - you have the right to do it, but establishments also have the right to not let you in. It's not about legality. It's about the choices of that establishment. They have a right to not allow you to do something, even if it's legal.
Those who know what e-cigs are have always given me an uninhibited "yes." Those who don't, I have explained it to, and then they give me an uninhibited "yes."
I have, in fact, never been told I couldn't vape.
Once I get that "yes," I vape politely. I don't take power-drags, like I do at home or outside. I take smaller drags, and hold them for just a touch longer, so the vapor is less. I don't go as far as stealth vaping, but I also don't let my vapor trail float into other people's space.
Doing this, I have never gotten a negative reaction.
People who don't know what vaping is, get to see me vaping, notice that it doesn't smell like cigs, and that what I'm vaping actually doesn't look quite like a cig. After getting permission, and vaping politely, they also see that it is non-invasive, and in their minds, they move me slightly outside of the "box" they put smokers in.
Smokers ask me where they can get one, non-smokers ask me what smells like cupcakes, and everybody's happy.
That's one way we can give vaping positive press.