What's a vaulted skull?
A chicken skull with a hole or a bulge at the top. It is found in some chickens with crests of feathers on their heads.
It tends to either have a hole in the top, or be thinner on top. So it does not protect the chicken's brain as well as a normal skull does.
Some chickens with vaulted skulls do just fine, but some die or have brain trouble from hitting their head on the coop, or from being pecked by another chicken. I think most probably do fine, because otherwise they would have all died by now