That is an odd leading theory.

Afaik the leading theory is people are born with it, and "it" can show in a lot of different ways, but a list of symptoms are very telling and there are clear patterns of behavior associated with it. Also there's a specific age when people start to notice autism in children most prominently.

From what I can tell, you can't "become" autistic.

And vaccines and autism only overlap because the age where autism tends to stick out in children falls bang in the middle of the age range where children get their vaccinations. So it lines up. But in a bunch of studies a direct correlation was never found, and neither was causation.