I am sure I read somewhere recently that only around 20% of cases of ASD would/could be related to vaccinations, so that would somewhat go against that theory.
However like you, I don't actually believe in a class discrepancy in regards to autism.
As although my friends and family would indeed represent a limited demograhic, since joining the world of special needs (DS1 is severly autistic, DS2 has Downs Syndrome), I can talk from experience that ASD does not favour any class in particular (I have met people through DS1's school, various support groups, etc, that I would never had the pleasure of meeting without said places and they indeed represent a wide section of society).
I heard someone say something that I found really eye opening once and that was 'Autism, coming to a family near you soon'.
As alarmist as that sounds, unfortunately it seems to be reality and no one should feel immune to the possibility of having their life or the life of someone close to them being affected by ASD, there is no magic shield, especially not class.