The 39-year-old wore jeans with heels and an oversized jacket to unveil her film Je Me Suis Fait Tout Petit.
She was joined by her co-star Lea Drucker and the film's director Cecilia Rouaud.
Vanessa and Johnny confirmed last month they have separated after 14 years together.
The Dark Shadows star is said to be handing over £100 million in a bid to keep the break up amicable for the sake of the couple's two children, Lily Rose, 13, and nine-year-old Jack.
After months of denials, a publicist for the actor said in a statement that the two "have amicably separated" and requested privacy.
There were numerous reports that the pair have been living separate lives for a while - though the Hollywood actor denied they had split at the unveiling of Tim Burton's film Dark Shadows in May.
Vanessa had also dismissed the reports as "rumours" during radio and TV appearances.
Johnny, 49, and 39-year-old Vanessa met in 1998 but never married.