For the last decade, celebrities have regularly replaced models on the covers of the style-centric monthlies. But Vogue’s decision to feature three models on its September cover — the fashion bible’s most important issue of the year — signals there’s been a shift in the symbiotic relationship between Hollywood and the fashion industry.

“Celebrities have become all too commonplace,” sniffs one New York-based designer. “I welcome the return of the model where the emphasis is on the clothes. It’s long overdue.”

The star imprimatur isn’t a guarantee of success in the fashion aisle. Vying for the same consumer hearts and handbags as established designers, Jessica Simpson, Victoria Beckham, Jennifer Lopez and Sarah Jessica Parker, among others, have helmed their own fashion lines — with decidedly mixed results.

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“Celebrities have become fashion moguls in their own right, but for it to work, all the planets have to align,” says Fern Mallis, creator of New York Fashion Week and now an international consultant. “It has to be the right fit between the celebrity and the brand.”

That’s something Parker learned after two less than stellar forays on to Seventh Avenue. She signed on to do a fast fashion (read: cheap) line for discounter Steve & Barry’s in 2007 that ceased after a year and a half when the retailer went bankrupt.

In 2010, she was named president and creative director at Halston Heritage. Two years later, she reportedly received a $3 million settlement for early termination of her contract because she did not see eye-to-eye with the company’s board.

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Perhaps the third time will be the charm: Earlier this year Parker introduced SJP, a line of shoes (under $500) in partnership with George Malkemus, president of Manolo Blahnik, whose shoes Parker popularized on “Sex & the City.” Available exclusively at Nordstrom, the collection features colorful stilettos that Carrie Bradshaw would love. Parker has expressed high hopes for this venture, telling one outlet: “This was different. I wasn’t a hired hand.”

Victoria Beckham’s had more success: The ex-Spice Girl launched her eponymous collection in 2007 with the backing of Simon Fuller and now does a brisk business at luxury emporiums Bergdorf Goodman and Saks Fifth Avenue. “She had big-time capital behind her, was very methodical about building her business and has worked hard,” says Teri Agins, whose new book, “Hijacking the Runway,” chronicles how celebrities have taken over the fashion industry and changed the business. “At this point, she’s really considered more of a designer than a celebrity.”

The same can be said of Mary-Kate and Ashley Olsen, whose fashion empire is reportedly worth more than $1 billion. Earlier this year, the former child stars won the coveted CFDA award for Accessories Designer of the Year, beating out industry darlings Alexander Wang and Proenza Schouler. They also won Womenswear Designer of the Year in 2012 for the Row, their minimalist chic line that is a perennial best seller at Barneys New York. “Everyone in the business takes them very seriously,” says Agins.

Jessica Simpson, too, has conquered the retail market, with a reported $1 billion in sales in 22 categories including sportswear, shoes and sunglasses sold in department stores including Macy’s. (Simpson’s annual take is said to be in the tens of millions.)

Earlier this year she headlined the Forbes Power Women summit, where she explained how she’s bested more established names in fashion, “I have been every size on the planet and I understand women,” she said. “There’s life beyond L.A. and New York. I understand middle America and their mindset.”

Perhaps the celebrity era is far from over.

“The celebrities started out as billboards for the designers, and have now turned the tables on them,” says Agins. “Depending on who they are, celebrities who know a lot about clothes can still have a lot of clout with consumers.”