I’m not sure what a vaulted skull is either, nor am I a chicken genetics expert, but I looked up crested chickens and found this on Wikipedia: “The extreme upright stand of the head feathers of many breeds, like the
Polish, can be attributed to a skull malformation, which is known as cerebral hernia or vaulted skull. Neurobiological investigations showed that this malformation alters the brain anatomy without any functional relevance.”
Here is the full article:
https://en.wikipedia.org/wiki/Crested_chickens