It should never happen. The vault should be padded and when learning a new vault the coach should be in real close, both to spot and intervene when things go awry. Accidents certainly can, and do, happen in gyms. But it is the gym's responsibility to make sure that the facility and equipment is safe, it is the coaches responsibility to spot according to needs.
Moreover there should be a system in place to deal with any injuries, here we have an injury report to fill out and a book to note incidents. Parents should be talked to directly after any accident, even if all looks well. Head bonks are a whole category by themselves, parents should be called and advised ASAP of the incident. Seemingly small head traumas can lead to bigger problems.
I am sure you kid was not messing around, but even of he was you still have the right to be informed.