Some people don't want to get caught in their hypocrisy. Vance likes to make broad sweeping claims, but it's all aesthetic-based pandering ultimately to cultural conservative eggheads out of touch with reality. And it will probably help cost Trump the election.
There are many, many Republicans that are childless. My aunt, for instance, is a lifelong Republican, and childless. The decline of birthrates in the developed world is something no goverment policy has been able to reverse, and I personally don't think it's going to be that much of a problem. I have faith that humanity will continue, as it has for the past aeons, and the decline in birthrates is a necessary outcome of women achieving greater autonomy and social equality, which are not at all bad things.