It will happen eventually. Here's how to handle it without being a jerk:

1. Just stop. No questions asked. "No problem" and put it away.
2. Don't argue. "It's just water vapor" "It's not smoke" "It's legal" — none of these matter. Someone asked you to stop. Just stop.
3. Don't sigh or roll your eyes. It's petty and makes all vapers look bad.
4. If you're in a public space where you think it's allowed, you can say something like "I thought it was OK here" — but if they say it's not, drop it. Move somewhere else.
5. Move somewhere else. There's always somewhere else you can vape. It's not a big deal.

Remember: every time you handle this gracefully, you're representing all vapers. Every time you argue about it, you're confirming someone's negative stereotype. Be the good example.