I don't think you really need to justify yourself. You can always turn the question around and ask why they are vaccinating...
We do vax, but not 100%. The nurse asked why and I just said we decided not to get those ones. And that was that.
I know first time round we got everything with DS and the truth was, I just hadn't thought about it enough to actually make a decision, so I just went along with the schedule. I assume if someone doesn't vax (or only partially) then they have obviously thought about it and come to a conscious decision.