You have to buy a concrete vault if you want to bury him in this cemetery."
Many cemeteries require either a vault or a steel casket. It's because wood deteriorates and collapses, leaving a divot in the lawn that has to filled in later. It has nothing to do with preserving the body although some morticians might make use of the rule to sell a fancier box.
FWIW, everyone in my close family has opted for cremation.
