My standard line: "It's not smoke, it's vapor. There's nothing in it that isn't also in food and medicine except nicotine, which carries no more health risks than coffee and has more benefits."
Then, if it's a private residence or business or anywhere smoking would be prohibited: "If you'd still like me to go somewhere else or stop, I'll be happy to. But I want you to know that nothing you know about smoking applies to what I'm doing."
If they tell me where a smoking area is: "No thanks. I don't want to expose myself to second hand smoke. Unlike what they do, what I do doesn't harm anyone and has no effect on anynoe else."
That usually works, but if they ask for details, I have citations stored in my phone and give them a reading list. So far, no one's cared enough to even take the reading list. If the conversation gets that far without them getting huffy, I've already won. If they end it by not caring what I have to say, having a team of scientists with conference presentations ready to defend me wouldn't have changed their outlook.