i have run into this already, and it's hilarious.
first, i assert to them that if vaccine theory works, then i couldn't possibly be putting their children at risk because their children are vaccinated. the only children at risk, therefore, are unvaccinated children.
usually, they "get" this but do a nice "yes, but" vaccines erradicate diseases!
so, second, i point out the fallacy that vaccination 'erradicates" a disease. small pox is a great example of this. in the past, everyone was vaccinated for small pox, and therefore no one came down with small pox. today, people are generally n ot vaxed for small pox, and now we're seeing a resurgence of the disease. therefore, vaccination doesn't erradicate disease (the disease still exists), it simply prevents wide-spread infection of the disease.
and everyone agrees and then says 'yes, but' vaccinations are a great advancement and i don't understand why a parent wouldn't want to give their child the great health benefits of vaccination.
to this, and third, i bring up the various assumptions in this. the first assumption is that the vaccinations are a great advancement--not everyone agrees with this theory. there are people who believe that other health theories are more practical and appropriate than vaccination theory.
then, i bring up that there are health risks associated with vaccines that might outweigh--to that parent--any benefits that a vaccine gives, assuming the parent believes that the vaccine gives any benefit.
and finally, i assert that just because a parent doesn't agree with these things doesn't mean that they are not considering their child's health in great depth--it's just that they think about health and medicine differently than the mainstream thought on the matter (vaccination).
and then they give up, because there's nothing more to say, honestly.