In an interview with People magazine, Vanderbilt, 92, revealed details about a past relationship that not even her son knew.
"When I went to [Miss Porter's School in] Farmington, I went through a brief so-called lesbian relationship with a girl in school," Vanderbilt told People.
"Cynthia, her name was, and she came once to visit my aunt in New York on holiday," she continued. "We had this sort of lesbian relationship and it felt so great."
Cooper, 48, revealed that he had not heard this story before, and noted that his mother had not included this story in the memoir that they co-wrote together titled "The Rainbow Comes and Goes: A Mother and Son Talk About Life, Love, and Loss."
"What? Hello! This is news to me," Cooper said. "You didn't mention this in the book, Mom."
The actress and socialite also noted that her prior understanding of homosexuality was partly shaped by the public gossip in the early 1930s surrounding her mother, Gloria Morgan Vanderbilt, which implied she was a lesbian.
Vanderbilt, who was 13-years-old at the time of the her own same-sex relationship, said she knew that it wouldn't last.
"This is before the thing I knew about my mother," she said. "I thought, 'No, this is something that's not really what I want.' It was very brief."
She's since noticed a shift in society's acceptance of same-sex relationships.
"It has nothing to do with whether it's two men or two women, it's love," Vanderbilt said. "And it's just like being married, it's what it is."