Normally the bodies are buried 6 feet below the box. This style stems from the early practice of piling stones on the buried body to keep animals from digging it up. The Romans made the box style popular and the Brits keep this style going. English immigrants into the Tide Water of Virginia area brought this style with them and it has filtered around the East coast and the deep South.
The only time I have ever seen bodies inside the brick vault is in more modern African American cemeteries. I was once told by a funeral director in Savannah that they put the bodies inside to save the money of the burial fee. This makes as much since as any other answer, so I’ll take it.
Hope this answers your question and thanks, I love questions.