vaping is very new and you have to keep that in mind. If someone asks you to stop: STOP! You can then immediately turn to them, show them the
device (even offer them to take a drag if they are a smoker) and hand them a CASAA card. Ask their permission to continue using the device now that they understand and if they say no say "Okay, thank you for the consideration and please look into the card I gave you." At this point, it'd be okay to include a small and polite tidbit of information about them, such as the lack of foul odors and ash - and even mention that they're used by millions worldwide for their benefits over smoking.
A rule of thumb that I like to remind my friends of which is very important to remember inside of privately owned establishments: Just because it's not a cigarette does not mean the owner can't tell you not to do it. The owner can also tell you to not listen to music in doors. The owner can also ask you to not wear a shirt with vulgar images on it. Your freedom doesn't express you the ability to do something others find offensive - even if they SHOULDN'T find it offensive - at least not in a private establishment.