What is the point of Vanity Fair? I need to know because a glossy selection of the “best” Vanity Fair portraits of slebs has arrived at the National Portrait Gallery, where it forms an eye-popping daisy chain of hardcore photographic buttering-up that obviously requires thinking about and probing. You should, for instance, see Arnold Schwarzenegger posing like a Nazi hero in T-shirt and skis atop a snow-clad mountain. Heil Arnold, indeed! Or the naked Gisele Bündchen sitting astride a big white horse, gamely pretending she isn’t having her crotch spiked by harsh stallion hair. If I am to have my stomach turned by all this high-class, full-colour fawning, then I am surely owed an explanation for it.

Like you, reader, I have had it up to here (I’m trying to touch the ceiling now) with goddamn slebs and their problems and addictions and mental illnesses and divorces and custody battles and trips to Africa and accidental overdoses and spells in rehab and flabby bottoms and corrective surgery and problem parents and the whole grotesque and disfigured rest of it. The worst of them turn up here: Diana and Madonna, Keira and Scarlett, Mick and Jennifer and Tom and Liza and Jack. So, I repeat, what is the point of Vanity Fair? If it exists merely to provide a posh gateway into a cesspit, how can that possibly be enough?

You probably know that Vanity Fair, the magazine, takes its name from Thackeray’s complicated satire on posh English society set in the era of Waterloo (he purloined the phrase from Bunyan). The novel overflows with lusty tales of husband-jumping and social verticality, too, but, unlike the magazine, it was energetically against all that. Thackeray, when asked, was clear about his purpose. He saw his characters as “abominably foolish and selfish” and set out to satirise a society characterised by hypocrisy and opportunism. The book’s subtitle, A Novel Without a Hero, was chosen not because we had entered the age of the antihero two centuries early, but because nobody in the story behaves well. Everybody disappoints.

More or less the same social types pop up in the magazine as popped up in the book. But, instead of becoming the focus of a nay-sayer’s cruel wit and having their balloon savagely punctured, the modern sleb is invited to step into that particularly plush and soothing bubble bath that is a photo-shoot by Vanity Fair’s star photographer, Annie Leibovitz. It was she who persuaded Scarlett Johansson and Keira Knightley to whip off their togs and disport themselves milkily across an expanse of black cloth in a supremely unctuous double-page spread shot in 2005. You can almost hear the slurping of the soft soap as it gently caresses the two blonde beauties between their egos.

As a trained art historian, I have the additional thrill of spotting the vague references made here to Manet’s Le Déjeuner sur l’herbe and Boucher’s notorious nude portrait of Louis XV’s mistress Louise O’Murphy, whom Boucher draped sexily across a sofa and persuaded to stick her bottom in the air. Leibovitz has Johansson mimic the pose and prove that her derrière is as plump and desirable as O’Murphy’s. Had our two modern beauties stripped down to this chipolata state in Thackeray’s world, it would have been to emphasise the bovine submissiveness of the modern actress, or to lament the graceless mistress role they have been forced to play by a sleazy society. But nothing as worthwhile happens here. All we are watching is a glamorous game of visual snap across the ages. The same occurs in Michael Thompson’s nude Julianne Moore, with its obvious nod to Ingres’s Grande odalisque. I doubt Leibovitz or Johansson are aware that O’Murphy was 14 when Boucher painted her, and that their recreation celebrates an act of rococo paedophilia.

Leibovitz is also the subject of a documentary biopic that has arrived at the ICA, made by her sister, Barbara. This embarrassingly emollient hagiography follows Annie around the world as she gets George Clooney to take off his jacket in front of a ring of half-naked models shivering in the sea, or cavorts around a French palace with Kirsten Dunst in Marie Antoinette gear. The increasingly theatrical effects Leibovitz prefers are explained by her growing need to bring something different to a portrait. But for someone who started out in San Francisco in the flower-power era, working for Rolling Stone and hounding Nixon, her transformation into a puff-master to the stars is dispiriting to behold. As she admits in the film: “The famous are winning.”

Indeed, they are. Vanity Fair has been around since 1914, and seems to have recognised from the off that slebs need to be buttered up. In my naivety, I had imagined that, in its earliest manifestation, the magazine harboured some satirical ambition, but no. Early in the show, James Joyce pops up sporting a dramatic eye patch, alongside a caption informing us breathily that “a new star has appeared in the firmament of Irish letters”, and that Ulysses is “the most important single volume of fiction written in our time”. At least the black-and-white portraiture of Steichen, Stieglitz and Berenice Abbott is superior to what comes later and features moments of true invention: Gloria Swanson shot through her veil by Steichen; a terrified Peter Lorre envisaged by Lusha Nelson, in 1935, with a circle of pointing fingers accusing him scarily of weirdness.

This first incarnation of the magazine ceased publication in 1936. When it returned in 1983, brought back by Condé Nast, it was a different vehicle running on different fuels. The presidents and pop stars, actresses and millionaires, artists and Shake-spearians, bike-racers and murderers who crowded into the relaunched Vanity Fair seem madly determined to flash their bits. Why did Demi Moore thrust her naked pregnancy so sensationally in our faces? Why did Margaret Thatcher sit for the loathsome Helmut Newton, even though he specialised in S&M fantasies? Why did Claus von Bülow pose for Newton in a sinister leather jacket while the wife he was accused of attempting to murder lay comatose in hospital?

“Vanity is truly the motive-power that moves humanity,” insisted Jerome K Jerome in The Idle Thoughts of an Idle Fellow, “and it is flattery that greases the wheels.” Did he have Vanity Fair in mind? A deadly combination of vanity and access has allowed the magazine to engineer a worrying complicity between the publication and its sitters. If I were a sleb, not only would I never say no to Vanity Fair, I might even pay them to put me in. I’m not talking here about the exposure the magazine gives you, I’m thinking of that soft, luxurious, creamy air of contented image-pampering Vanity Fair ladles out so generously.

Nietzsche called vanity “the most vulnerable and yet most unconquerable of things”, and I suppose that is why the defeated George Bush pops up here, surrounded by Dick Cheney, Donald Rumsfeld and Condoleezza Rice, all attempting to look as tough as the Earps did at the OK Corral. The Leibovitz film actually begins with a world-class display of vanity, as a parade of famous faces, from Hillary Clinton to Mick Jagger, begin chanting Leibovitz’s name as if we were in the presence of royalty.

“A vain man may become proud and imagine himself pleasing to all when he is in reality a universal nuisance,” Spinoza observed. The same, I suggest, goes for a vain woman – and particularly Madonna, who has appeared on the cover of Vanity Fair a record nine times, and was most recently seen saving Africa, in the company of Archbishop Tutu and others, in last summer’s grotesque continental special edited by Bono. What is the point of Vanity Fair? That’s easy. Vanity Fair is there to prove that one of our vices is unstoppable.

Vanity Fair Portraits, NPG, WC2, until May 26; Annie Leibovitz: Life Through a Lens, ICA, SW1, until March 16

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