Vacth was born on 9 April 1991 in the 12th arrondissement of Paris. She grew up in the Paris suburb of Maisons-Alfort. Her father is a truck driver and her mother is an accountant. Her surname is of Lorrainian origin.
Vacth began her modeling career at the age of fifteen after she was scouted in H&M and started acting at twenty. She played "Tessa" in Cédric Klapisch's film My Piece of the Pie. In 2011, she succeeded Kate Moss as the face for Yves Saint Laurent perfumes and the Chloé brand.
She lives in Paris with her boyfriend, Photographer Paul Schmidt, and their son Henri, born in the spring of 2014.