This article is from the James Taylor FAQ, by Joel Risberg (email@example.com) with numerous contributions by others.
People often suggest that the song is about JT, but according to Carly
herself, it was about Warren Beatty. Other theories survive, though.
From JD Delorme (firstname.lastname@example.org):
The rumor going 'round when I was but a lad was that "You're
So Vain" was about Mick Jaggar - whom she dated for awhile - and who heard
it and knew it was for him. Then he begged to be allowed to do background
vocals - a little twisted humor - sort of the ultimate irony for the whole
idea that "you probably thing this song is about you". Of course - it may
well have been more urban folklore, too.
From Charles R. Center (CENTERCR%DFL@pcmail.usafa.af.mil):
Saw an interview once, and she said it was about Beatty, and acknowledged
that alot of people mistakenly attributed it to JT.