You're so vain, you probably think this song is about you
You're so vain, I'll bet you think this song is about you
Don't you? Don't You?
Well...it is about him, isn't it?
Update: I wonder...maybe it is that the 'you' is plural. So she's singing about multiple guys and each one probably thinks the song is about him, when it is about all of them. Different terms for 3rd person singular and plural would help here.