A vaulted skull is also called a cerebral hernia. At hatching the skull is not closed on top, and the brain and other soft tissue form a large knob atop the head, but under the skin. In some cases the skull eventually grows together over the knob. In other cases it remains open.
Some say that crests grow larger on birds with the vaults; others say it does not matter. I do not know of anyone who has actually done a wide-scale study to determine whether there is anything other than anecdotal "evidence" for either viewpoint. Vaulted skulls are required in polish; not required in silkies. Most silkie breeders that I know of do not breed either for or away from the vault.