Areas with the highest vaccination rates will also have the highest rates of people seeking medical treatment along with availability of medical treatment, so naturally the rates would be reported higher there.
Bumpkin Podunkville where there is one country hospital, no one gets vaccinated, and there isn't a mental health facility will naturally have fewer reported cases of autism because people won't have someone to go to in order to be diagnosed.
On the other hand, the largest study to date- looking at over 2 million people diagnosed with autism, found that there is an inverse correlation- those who are vaccinated have a smaller chance of being diagnosed with autism.
http://www.gopbriefingroom.com/index.php/topic,312973.0.html