Safe or not for smokers, non-smokers should certainly not have to inhale the same substances that smokers do. I find it alarming that e-cigarette marketing managed to convince smokers that what gets exhaled is just "water vapor", and thus they can now smoke anywhere they want.
I don't understand. In my experience these vaporizers don't have any sort of smoke that lingers around and there is no unpleasant sensation associated with breathing around ecigarette users like there is with tobacco. Even if there is more than water vapor in there, I feel like it's very unlikely that I'd be getting any sort of significant dose from their exhalations, so why should I care if they smoke inside?
I'm personally not comfortable inhaling your random unknown vaporizer gases. Many chemicals are dangerous at levels you are not able to smell, or indeed have no odor.