The text is a play on the word 'vain', suggesting that a vain person would assume a tagline (or song lyric) is about them due to their self-centeredness. It's a reference to Carly Simon's song 'You're So Vain', where the subject of the song is accused of being so vain that they would think the song is about them. This exploits the characteristic of vanity for humorous effect but does not rely on different meanings of a word or similar-sounding words.
You ' re so vain / I bet you think this tagline ' s about you