Also, why do you have to have a vault for burial?
A burial vault is used to line the grave before placing the casket or coffin in it, so as to prevent the ground above the casket from sinking in. Cemeteries usually lay emphasis on the vaults and grave liners to reduce their burden of maintenance required to keep the ground on level.
Considering this, do caskets go straight into the ground?
A burial vault encloses a casket on all four sides, the top, and the bottom. Modern burial vaults are lowered into the grave, and the casket lowered into the vault. With a burial liner, the coffin is lowered directly onto the earth. The burial liner is then lowered over the casket.
Most cemeteries require a burial vault or liner for maintenance purposes when the ground settles around the casket. According to the Federal Trade Commission's funeral rule, it is illegal for funeral providers to claim a burial vault or liner will keep water or dirt from getting into the vault or liner.