Having cerebellar hypoplasia does not mean that he will not have a good quality of life. There is a very recent thread on this. I would encourage you to watch the video that LDG posted.
Now I have to figure out if keeping this kitten is a bad choice due to the quality of life he will not have.
But it might have made no difference - he might have had this problem anyway. My feeling is that he would. She had the vaccine late in pregnancy and the litter mates are not affected and you haven't said any of them have any signs of illness. Causes other than the vaccine include asphyxia at birth, viral infection before you took the mother in, malnutrtion during pregnancy, poison and injury. My own feelings lean towards asphyxia or injury since the other kittens don't appear to have a problem. Yes the vet shouldn't have vaccinated here when it was possible she was pregnant, but what's done is done. If it was me I think I'd change vets though. He's been careless once, he might be careless again. Note that careless isn't the same as culpable.this could have been inflicted on the kitten and avoided