"Do something about it"? So against the voices of actually autistic people, you would like to start preventing or curing autism?
Sorry to disappoint you, but it isn't going to happen. Autism isn't an illness or a "problem", it's genetic and an autistic person is just as worthy or living as their authentic selves as an allistic person.
You can't cure autism anymore than you can cure having brown hair or being tall.
We cannot effectively compare the rates of autism 100 years ago to now, because 100 years ago autism wasn't even known about. Children who were autistic- and we know they did exist- were either high support needs in which case they were institutionalised, or low support needs in which case they were just seen as a bit quirky or naughty and in need of punishment.
We can't even compare the rates of autism 20 years ago, because the diagnostic criteria have changed a lot as we've recognised that autism can present very differently in different people. If you went and got an autism diagnosis tomorrow, on paper, that's another person with autism on the record. You didn't become autistic because of plastics or toxins. You were always autistic. It was just not noticed because of a lack of awareness.
However, we can absolutely say that as long as we have records going back, we have records of people who would undoubtedly meet the criteria for autism diagnosis today, despite the lack of vaccines. Just as we know that thousands of children died from preventable diseases.