No matter who the person being referenced in the song “You’re so vain” is, the song *isn’t* about them; the song is *about* the singer and their feelings. Hence the accusation “You’re so vain, I bet you think this song is *about you*” is an accusation that the person being referenced is still self-centeredly thinking that things are ‘about  them’, rather than centering the feelings and needs of the singer.