I vape anywhere & everywhere and have never been confronted. If I ever am, I plan to say it's a personal vaporizer (not lying) and see if they'll let it go.
If not, I'll explain as necessary and hand them a V4L business card. Of course, I'll hand it to them so they see the back first.
If they still press the issue, I'll just comply (assuming someone in authority). The last thing we, as vapers, need is negative attention. Besides, it is a business' right to prohibit whatever they wish.