A video clip of Saturday night's duet has been viewed nearly 110,000 times since Swift posted it on YouTube Monday.
"Before I bring out my special guest, I have this question that I've always, always had," Swift told the audience. "I think a lot of you have probably had this question too, always wondered what the answer is. Who is this song, 'You're So Vain' by Carly Simon, written about?"
Swift, 23, then brought Simon, 68, on stage and the pair sang the song after which the younger singer instructed the crowd, "Ladies and gentleman, give it up for one of your own, Carly Simon!"
Although neither Swift nor Simon discussed on stage who the song might be about, the Los Angeles Times said Mick Jagger, Kris Kristofferson, James Taylor, Warren Beatty and the former Cat Stevens have long been rumored as potential subjects.
"Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!!" Swift tweeted after the concert. "Unreal. I could not possibly look up to her more."