A genetic metal efflux disorder could cause children to accumulate more toxic metals than their peers. With regressive autism, could it be possible that the mother's Cu:Zn ratio was fine, but the child inherited one copy of the Wilson's Disease gene from the father, and therefore had no symptoms until being given several vaccines which exceeded the child's metallothionein reserves? David Kirby's lecture series reports that the gene inheritance pattern for the MtD link in autism seems to be strongly paternal. 
The fraternal versus identical twin studies show a strong genetic association, but the incidence of autism occuring in both identical twins is not 100%. That proves environmental factors are also important. While identical twins typically would have an almost identical environment, some possible variables are diet and health status at the time of vaccination.
Diets can affect zinc levels and even identical twins may prefer different foods or eat different amounts of foods. Also, all infants and children are at risk for zinc deficiency, and severe diarrhea can cause zinc deficiency in people of any age.  So infants and children with severe diarrhea would probably have VERY low levels of zinc, which results in low levels of metallothionein. Therefore, if one twin was moderately ill or not yet fully recovered from an illness at the time of vaccination, that twin might be more likely to suffer an adverse reaction than the healthy twin.
Since all infants and children share this risk of a zinc deficiency, even children with no genetic susceptibility could be also be at risk of suffering a severe vaccine reaction if their zinc levels are very low and they are given too many vaccines for their weakened immune systems to handle.