In a recent interview with PEOPLE and Entertainment Weekly editorial director Jess Cagle, Cooper, 48, learned that his mother, 92, was once in a same-sex relationship.
"I, myself, when I went to [Miss Porter's School in] Farmington, [Connecticut] I went through a brief so-called lesbian relationship with a girl in school," Vanderbilt revealed after Cooper discussed coming out to the author.
In response, the CNN anchor appeared shocked, saying, "What? Hello," and adding, "this is news to me. You didn't mention this in the book, Mom."
"Cynthia, her name was, and she came once to visit my aunt in New York on holiday," continued Vanderbilt. "We had this sort of lesbian relationship and it felt so great. It felt so good and yet I thought, 'There's something about this,' and this is before the thing I knew about my mother. I thought, 'No, this is something that's not really what I want.' It was very brief."
Vanderbilt revealed that she was 13 at the time of the relationship, and said she now realizes, "I think almost everybody goes through at one point … of course, the thing is, now we realize there's no difference. Love is love."
Watch more of The Jess Cagle Interview with Anderson Cooper and Gloria Vanderbilt all this week on People.com
Vanderbilt said her understanding of homosexuality was largely shaped by the public gossip surrounding her own mother, Gloria Morgan Vanderbilt. Cooper, who came out as gay in 2012, explained that during a 1932 custody trial Gloria Morgan Vanderbilt was "accused" of being a lesbian.
"That word wasn't spoken. The press wasn't supposed to write about it," Cooper said. "My mom knew that something terrible had come out in the court, but she didn't know what it was and she didn't know what being a lesbian meant at the time, obviously."
For more on Anderson Cooper’s extraordinary relationship with his mother Glorida Vanderbilt, pick up a copy for this week’s PEOPLE magazine on newsstands Friday.
Added Vanderbilt, "Then of course, when I heard what it meant, it affected me so much because I thought this is something so terrible. This is like a terrible brand, like a terrible stamp. Is it something that maybe I have inherited? That was very difficult for me growing up to try to figure it out."
It seems time has shaped Vanderbilt's understanding, however, as she said, "It has nothing to do with whether it's two men or two women, it's love. And it's just like being married, it's what it is."
Cooper and Vanderbilt's relationship will be featured in an April 9 HBO documentary Nothing Left Unsaid and, before that, in a memoir, The Rainbow Comes and Goes: A Mother and Son Talk About Life, Love, and Loss, due out April 5.