Never heard of a concrete requirement.
The casket typically goes into a metal box for that purpose.
I suppose you could use concrete.
In most places there is no law requiring a burial vault. But they are often required by cemeteries. They prevent the ground from sinking in when heavy equipment is driven over the grave, or because of subsidence. This makes maintenance easier for the cemeteries.
Concrete vaults are very common. Some brands are painted a metallic color. Vaults can also be made of metal, fiberglass, or plastic.