Lots of factors going on here other than vaccines (the only control studies out there show no connection - apparently the onset of the first symptoms of autism often coincides with the age when vaccines are customarily given - post hoc proper hoc strikes again.) A few that come to mind:
The definition of "autism" has expanded dramatically. When I was a preteen/teenager I knew a couple of families who had autistic children. In those days, it wasn't a question of "poor social skills" or being a little geeky - one of those boys is 40 and still living with his parents, and when they die he'll have to be institutionalized. Nonverbal, no interaction with anyone (even his parents), spends his time being hyperfocussed on shadows on the wall or cracks in the sidewalk . . . it's just a dreadful situation and there doesn't seem to be anything anybody can do for him. But there is absolutely no question that he's autistic . . . now you see kids labelled "autistic" who are just, as you say, a little on the weird side of normal. Shoot, if that's the criterion you might as well go ahead and declare my entire family autistic . . . we used to be known as "eccentric."
Of course, in those days there was also a stigma attached to autism because it was thought to be a psychiatric problem caused by a cold, unemotional mother. Which tended to hold down the number of cases, nobody wants to be blamed for such a catastrophic problem. Now, the schools get money and parents can get SSI for children with the diagnosis. Which tends to increase the number of diagnoses.
Good summary. I think there is both an increase (over the last 30 years) in the diagnosis of genuine autism cases that might not made it into the statistics in the past. However, there's also an increase in autism diagnoses for "any kid who's a bit different."
I'm worried about sending Pat to Sunday School next year, because he talks to space ships on his pocket calculator and thinks he's invisible!