Even before the last part of the serial was published, critics hailed the work as a literary treasure. Although the critics were superlative in their praise, some expressed disappointment at the unremittingly dark portrayal of human nature, fearing Thackeray had taken his dismal metaphor too far. In response to these critics, Thackeray explained that he saw people for the most part "abominably foolish and selfish". The unhappy ending was intended to inspire readers to look inward at their own shortcomings.
The subtitle, A Novel without a Hero, is apt because the characters are all flawed to a greater or lesser degree; even the most sympathetic have weaknesses, for example Captain Dobbin, who is prone to vanity and melancholy. The human weaknesses Thackeray illustrates are mostly to do with greed, idleness, and snobbery, and the scheming, deceit and hypocrisy which mask them. None of the characters are wholly evil, although Becky's psychopathic tendencies make her come pretty close. However, even Becky, who is amoral and cunning, is thrown on her own resources by poverty and its stigma. (She is the orphaned daughter of a poor artist and an opera dancer.) Thackeray's tendency to highlight faults in all of his characters displays his desire for a greater level of realism in his fiction compared to the rather unlikely or idealised people in many contemporary novels.
The novel is a satire of society as a whole, characterised by hypocrisy and opportunism, but it is not a reforming novel; there is no suggestion that social or political changes, or greater piety and moral reformism could improve the nature of society. It thus paints a fairly bleak view of the human condition. This bleak portrait is continued with Thackeray's own role as an omniscient narrator, one of the writers best known for using the technique. He continually offers asides about his characters and compares them to actors and puppets, but his scorn goes even as far as his readers; accusing all who may be interested in such "Vanity Fairs" as being either "of a lazy, or a benevolent, or a sarcastic mood". As Lord David Cecil remarked, "Thackeray liked people, and for the most part he thought them well-intentioned. But he also saw very clearly that they were all in some degree weak and vain, self-absorbed and self-deceived."
The work is often compared — for instance by John Carey — to the other great historical novel of the Napoleonic wars, Tolstoy's War and Peace. While Tolstoy's work has a greater emphasis on the historical detail and the effect the war has upon his protagonists, Thackeray instead uses the conflict as a backdrop to the lives of his characters. The momentous events on the continent do not always have an equally important influence on the behaviors of Thackeray's characters. Rather their faults tend to compound over time. This is in contrast to the redemptive power conflict has on the characters in War and Peace. For Thackeray, the Napoleonic wars as a whole can be thought of as one more of the vanities expressed in the title.
In the original illustrations, which were done by Thackeray, Becky is shown behind a curtain when Jos dies, with a vial in her hand; the picture is labelled "Becky's second appearance in the character of Clytemnestra" (she had played Clytemnestra during charades at a party earlier in the book). Joseph's death appears to have made her fortune.The suggestion near the end of the work that Becky may have killed Jos is argued against by John Sutherland. Although Becky is portrayed as having a highly dubious moral sense, the idea that she would commit premeditated murder is quite a step forward for the character. Thackeray was a fierce critic of the crime fiction popular at the time, particularly that of Edward Bulwer-Lytton. These lurid and sensationalist accounts—known as "Newgate novels"—took their inspiration, and sometimes entire stories, from the pages of The Newgate Calendar. What Thackeray principally objected to was the glorification of a criminal's deeds; it therefore seems strange that he would have depicted Becky as such a villainess. His intent may have been to entrap the Victorian reader with their own prejudices and make them think the worst of Becky Sharp even when they have no proof of her actions. The trio of lawyers she gets to defend her from the claims, Burke, Thurtell, and Hayes, are named after prominent murderers of the time, although this may have been a tease or further commentary aimed at the legal profession.
Though Thackeray does not settle definitively whether Becky murders Jos, such a development could be seen as in keeping with the overall trend of character development in the novel. The tone of Vanity Fair seems to darken as the book goes on. At the novel's beginning, Becky Sharp is a bright girl with an eye to improving her lot through marrying up the social scale; though she is thoroughly unsentimental, she is nonetheless portrayed as being a good friend to Amelia. By the novel's end she has become an adulteress and is suspected of being a murderess. Amelia begins as a warm-hearted and friendly girl, though sentimental and naive, but by the story's end she is portrayed as vacuous and shallow. Dobbin's infatuation with Amelia is a theme which unifies the novel and one which many have compared to Thackeray's unrequited love for a friend's wife (Jane Brookfield). Dobbin appears first as loyal and magnanimous, if unaware of his own worth; by the end of the story he is presented as a tragic fool, a prisoner of his own sense of duty who knows he is wasting his gifts on Amelia but is unable to live without her. Whether Thackeray intended this shift in tone when he began writing, or whether it developed over the course of the work's composition, is a question that cannot be settled. Regardless of its provenance, the novel's increasingly grim outlook can take readers aback, as characters whom Thackeray—and the reader—at first hold in sympathy are shown to be unworthy of such regard.
It has been claimed the character of Becky Sharp is based in part on Thackeray's maternal grandmother Harriet Becher, but according to Peter Shillingsburg she is said to have been the model for Miss Crawley. She abandoned her husband and children when she eloped with Captain Charles Christie. In 1806 shortly after the death of Christie and her husband she married Edward Butler, another army officer. Thackeray lived with his grandmother in Paris in the 1830s and again in the 1840s.