I interpreted this as where the medical students at Vanderbilt had gotten their bachelor’s degrees.
We do not have any MDs in the family, but I do have three very close relatives who got or are currently getting some sort of graduate degree (master’s or DVM or PhD) in a biomedical field at a well ranked university. All of them report a very similar wide range in terms of where the other students in the program got their bachelor’s degree. A doctor I know summarized this as “all over the place”.