As soon as you are in a relationship with someone who has a bigger audience than you, you become 'someone's girlfriend,' Vittoria Ceretti told French Vogue last year. The Italian model knows what she's talking about. She's been dating Leonardo DiCaprio for a few years, but she has a career that began long before her relationship with the Hollywood star. On Thursday, she turned heads again at the Venice Film Festival, where she arrived in a black…