I agree that it's likely that autism is multi-factorial. However, if vaccines were even a part of the story - more precisely, if being vaccinated made one more likely to develop autism, even though only slightly and even though lots of other factors were also involved - then there would be a correlation between being vaccinated and developing autism. There isn't, as far as we know - and it's a very well-studied question by now, so by ordinary scientific standards, we do know.
I suppose that if there were some way that the prevalence of vaccination in the population could make the population as a whole more likely to develop autism, without the actual individuals being vaccinated being at extra risk compared to others in the population who were not vaccinated, then that isn't ruled out. E.g., if somehow the manufacturing of vaccines were producing an air pollutant to which everyone is exposed, that would do it. Seems far-fetched, though.
