Another thing about Vanity Fair that the new movie will have trouble preserving (no, I haven't seen it yet) is the narrator, that intrusive, ironic-yet-judgemental voice that breaks in constantly to offer us another perspective on the story. Unlike some of the writers who imitated his narrative intrusions -- like the early Trollope -- Thackeray isn't exactly writing in his own voice; it's been pointed out that the narrator of Vanity Fair makes some references to his wife and friends that indicate that he's not Thackeray. Again, the most logical comparison is to War and Peace. The narrator/historian in that novel isn't exactly Tolstoy himself, and so the novel comes off as a two-level story: first, the actual fictional story, and second, the story of the historian trying to show how these small personal stories fit into the big conventional history of great men and battles. In the Waterloo section of Vanity Fair, Thackeray does the same thing -- sets his characters' stories within the context of yet another Napoleon screwup. But the narrator of Vanity Fair isn't primarily a historian; he's a novelist, and he's mostly concerned with explaining and mocking the conventions of narrative fiction. Those are Thackeray's two levels: a story about people, and a story about a guy who's writing a novel about those people. As the story goes on, the narrator clues us in on what a lot of lies and over-simplifications go into creating the conventional categories of a novel; how, for example, to make someone like Amelia Sedley a "heroine," you have to overlook the downside of her supposedly good qualities.
This isn't the kind of authorial intrusion you got in most earlier novels, because most novels before Vanity Fair tended to present themselves as "histories." The novel was supposed to be an outgrowth of historical writing, particularly when (as with Vanity Fair) it incorporates historical events; that's why Walter Scott's novels often start with a blurb telling us that what we are about to read is a long-lost historical manuscript, just to get us in the right frame of mind to suspend our disbelief. Thackeray doesn't want suspension of disbelief, at least not all the way through; he's pulling back the curtain and exposing all the ways that novels manipulate us. If we get to the point where we have no idea what to think of the characters any more -- and where we don't know what to make of the narrator (does he mean it when he tells us what to think of a particular character, or is this more irony? who's speaking here, Thackeray or the narrator?) -- then Thackeray has us right where he wants us: he wants us to be distrustful of the way fiction over-simplifies character and action, just as Tolstoy wants us to question the way historical writing over-simplifies history.
In a way, there's something irritating about all this irony, all this ambiguity about what we're supposed to think of the characters and the story. Sometimes I get the feeling that Thackeray uses irony as a way of copping out, of not taking a moral or philosophical stand: if we don't know what he wants us to think of Becky, and if we can't even be sure whether he (or the narrator) means it when he tells us what to think of her, then Thackeray doesn't have to commit himself to any fixed position on the morality of what she does. Because of all this proto-postmodern irony, Thackeray isn't open to the same charges of sentimentality and banality that can be levelled at Dickens (when Thackeray's narrator says something sentimental or otherwise Victorian, we can't quite be sure whether he means it), but he is open to the charge of emotional reticence, of not committing to his story or characters. And if the author can't commit himself, it's hard for the reader to do so -- which means that I can read through Vanity Fair and find myself wondering what the point of all this is, beyond the purely satirical or parodic points that became all-too-clear several hundred pages ago. That said, I think Vanity Fair is a masterpiece. But sometimes it can seem like a pointless masterpiece, if there can be such a thing.