Clearly she had some kind of unofficial contract for Prada besides being the face of the Candy perfume. Vogue called her a Prada ambassador. She attended Miu Miu shows in Paris almost every season, and barely attends any other fashion show. Also, even though she indeed wears many designers she was always in either Miu Miu or Prada for her most important appearances (big city premieres and award shows).
When she wore Vuitton it was still under Marc Jacobs. A pink dress at the Venice film festival from their resort 2011 collection; A gown from the fall 2010 collection for the Inglorious Basterds premiere in Cannes; And a gorgeous blue custom-made gown at the Cannes premiere of Grand Central that was inspired by one of Marc's final collections, that is fall 2013 where she sat front-row. She always looked good in Marc Jacobs's deisgns, for sure, but never wore anything by Ghesquière.
I completely agree that Camille is a fantastic stylist. I love what she's done with Adele Exarchopoulos LV looks. More than her great pairing with Prada, it's the great collaboration with her sister that made Léa's style one of the very best among actresses. Yet I still think that Nicolas universe at Vuitton doesn't suit Léa's pin-up, slightly old-hollywood glamour style.
I'm disappointed because not only Prada gave her prime samples from the collections and designed stunning custom pieces for her. But she could also wear other designers. LV, and French fashion houses in general, seem to be very restrictive.