Mr. Vaccarello founded his own brand, in 2009, but will suspend that line to “devote himself fully to Saint Laurent,” according to a spokesman for the brand. He will be responsible for women’s wear, men’s wear, the couture line revived by Mr. Slimane and the image of the brand.

His name first surfaced as a possible replacement for Mr. Slimane in January. He deflected the gossip as “a rumor” in March at his last show during Paris Fashion Week, which was marked by a combination of body-baring asymmetric lacing and leather that reflected his penchant for 1980s rock ’n’ roll, and that may connect his aesthetic to that of Mr. Slimane’s Saint Laurent line. (Ms. Versace once characterized Mr. Vaccarello’s signature look as “cool and sexy.”)

Certainly, he is not expected to carry out another wholesale reinvention of the house in the manner of Mr. Slimane, who redesigned not only the stores but also the furniture in the stores; photographed the ad campaigns himself; and dropped the “Yves” from the name of the ready-to-wear line. Unlike Mr. Slimane, who based himself in Los Angeles, Mr. Vaccarello is based in Paris.