The song tells the story of a lover who is as attractive as he is faithless and arrogant, and ever since Carly belted out the words, she's been asked in interviews all over the world to reveal the identity of the man who inspired the lyrics.
The refrain of the song goes, "You're so vain, you probably think this song is about you."
Simon, now 58, has always said she'd never tell who the song really is about and for decades, it seemed that was a principle that could never waver.
But there's something else Simon holds very dear as well: Martha's Vineyard, where she lives. Simon has agreed to tell the secret identity from "Vain" to the highest bidder in an auction benefiting Martha's Vineyard Community Services, which provides child care, counseling, substance abuse treatment services and visiting nurses to the Island community.
Simon's secret went for $50,000 at Monday night's auction in Edgartown, which raised a total of about $500,000 with the help of other items donated by other Vineyard celebrities including Walter Cronkite, Ted Danson, Mary Steenburgen, Diane Sawyer and Mike Nichols.
And who was the highest bidder? NBC Sports president Dick Ebersol.
If $50,000 seems a little low for such a closely guarded bit of music folklore, consider this: there's a catch.
Ebersol had to promise not to tell anyone else.
The consolation prize: Simon, famous for avoiding crowds, has invited the winner to her home for a private lunch of peanut butter and jelly sandwiches, vodka on the rocks, and a performance of the song, before telling her new confidant who it is about.
Several male celebrities have long topped the speculation list of who "You're So Vain" is about.
They include her ex-husband, folk singer James Taylor, and three other men she once dated: actor Warren Beatty, singer and actor Kris Kristofferson, and Mick Jagger, who sang backup on the song.
The guessing game over the song has alternately flattered and irritated Simon, who told Diane Sawyer in 1990: "Who cares? I can't believe people care, you know? It was a riddle a long time ago and it's best, as all those riddles are, it's best unsolved."
In 2001, Simon brushed off CBS News Correspondent Rita Braver's request to solve the mystery, saying: "I could never really solve it because if I did, then no one would have anything to talk to me about."
Somehow, that seems unlikely.