When people start going on about "what they put in vaccines" I grab any processed food item I can find, hand them the box, and ask them to read the ingredients out loud.
Usually they can't pronounce most of the big words, and most of the time they realize what I'm doing and stop trying to read and get super defensive/offended.
If you get offended by me asking you to read ingredients in your kids pop tarts, I can't imagine how you react to an actual problem.