The hard part, I suspect, might be distinguishing between someone who actually has a medical reason for not vaccinating, and someone who thinks "his older brother is on the spectrum" counts as a medical reason.

Yes, a bunch of anti-vax parents will out themselves as soon as you ask "are you and your child vaccinated against whooping cough?" or mention that you plan to get a flu vaccine. And eliminating those children and their parents from your social group might be enough to reduce your child's risk of infection significantly.