Because Vanity Fair is a satire, it is by definition an exploration of the moral and ethical questions of its time. At the same time, satire is a conservative genre, in the most basic sense of that word: it seeks to conserve and preserve the cultural traditions of the past in the face of modern erosion. Thackeray's disparaging eye ranges over rampant materialism, snobbery, and the brutal internal logic of the social hierarchy. The rigid social distinctions of a bygone era are being muddied by the influence of wealth and the desire of the newly moneyed for upward social mobility.
Becky's act of freeing Amelia from George's memory and letting her marry Dobbin is completely kind and selfless. That Becky is capable of such things means that she is correct about simply needing an income to be a "good woman."
In the novel, almost none of the characters who act badly or do wrong are punished or get any kind of comeuppance. This makes the novel a very dark satire indeed, as it gives us a picture of a world with no morality, either private or public (legal).