This is true. Many of the kids in my neighborhood who would have been considered autistic today were simply termed "slow" back then. However there was one kid in the neighborhood that I heard my parents refer to as autistic. He was completely non-verbal and didn't interact with anyone, not even his parents. So you're right, a comparison would be useless as only those at the very far end of the spectrum were diagnosed as autistic back then.
I don't think a comparison would help. Years ago, autistim would have been lumped in with mental retardation. I personally think it is genetic, possibly with enviromental factors.
You're assuming that vaccines are a new thing. They're not. I recently read a book that's 150 years old that mentioned vaccines. Surprised me. I knew they weren't a new thing but hadn't realized they were that old either. The culprit is supposed to be thimerasol used as a preservative in the vaccines. Thimerasol also was used in contact lens solutions 20+ years ago. It is my understanding that thimerasol is no longer used because of the controversy, and hasn't been for many years.
I still wish they'd get it figured out. I tend to think it's environmental because 3 women who spent their pregnancies in this house (myself, my daughter and my babysitter) all gave birth to autistic children. What is it? The soil, the water, the agricultural chemical plant 5 miles due north? Or maybe it's genetic and just a concidence about the babysitter. Who knows?