I understand how you feel.
With both my tiny prems I was the same in the end I delayed vaxed...I waited till there corrected age or even after in many cases.
It is hard when they are so small (DS is 6.8kg (14lb) at 17mths) and they are getting the same dose as a bigger baby.
I am not comfortable with them not getting it at all so delay vaxing was my compromise...my GP wasn't necessarily happy about it but he has accepted it.