usedto, I appreciate that you cannot KNOW to a decimal point, and none of us can know if our child will be the '1 in.....'
However, if you add up all the risks from all the diseases vaccinated against, and look at the available literature on vaccine damage, you can arrive at a rational comparison of risks.
So, say - bear in mind i have NOT done this exercise, but you could - you find out that, added together, the risks (just to your child, not to anyone else ion the population e.g pregnant women, the immunocompromised etc) from the diseases that are vaccinated against are that 1 in 50 children will suffer some kind of serious lasting harm, ranging from deafness or blindness to brain damage or death.
You can then look at the recorded risk of vaccine damage - which could, again plucking a figure out of the air, be 1 in 20,000.
You could then ask the common sense question: do I believe that vaccine damage is 400x more under-reported - ie that for every reported / acknowledge case there are 399 that are not - than harm from measles / mumps / rubella (bearing in mind that some mild brain damage etc from these diseases may also not have been reported)?
You cannot absolutely KNOW that the risk to your child is 1 in 50 vs 1 in 20,000 - but you should be able to arrive at a point where you either know that one riosk is so much larger than the other that under-reporting can make no substantive difference, or you arrive that the point where you feel that both risks are, given potential under-reporting, equal, and then go for the toss of a coin.