It's a huge gray are within the industry, articles have been written about it in trade magazines. You can ask anyone to leave but then if they file a suit or something you're basically bankrupt fighting it whether you win or lose. And no offense to anyone here who vapes, but the attention whores who vape indoors simply to incite something are exactly the type that file suits.
I have had one guy walk in smoking a real cigarette. I asked him to put it out, he walked outside, threw it on the ground and stomped it. I had one guy light up a cigarette right in front of me while he was hollering at me about how good my food was. (Yes, he was actually angry at me because my food was good.) I asked him to put it out and he didn't so I told him that it was illegal to smoke indoors and that I would call the police if he didn't extinguish it. Then he did the same as the other guy, walked outside, threw it on the ground, stomped it. He came back in after that to finish berating me for my excellent food quality. (He had obvious mental issues and had scared my staff, which was the reason they called me up there to deal with him.)