Not that Amelia Sedley is any great shakes as a character; she is, as Thackeray describes her, selfish, deluded, and simpering. She's not much of a heroine, but as Thackeray more or less admits at various points, she's not supposed to be. Amelia is the kind of woman who is idealized in most novels -- she is virtuous, she is faithful to her husband and then to his memory, she is devoted to her son -- but these qualities, which most novelists would celebrate, are shown by Thackeray to be faults, and pretty big faults too. Her devotion to George's memory isn't noble, and it isn't sweet; it's selfish, and Captain Dobbin's devotion to Amelia is shown to be foolish. When they finally let go of their delusions and get together, not for love but just because it's better to have each other than no one, Thackeray makes it clear that what would be the big love scene in another novel is transformed into a bittersweet little scene of two people settling for what they can get:
The vessel is in port. He has got the prize he has been trying for all his life. The bird has come in at last. There it is with its head on his shoulder, billing and cooing close up to his heart, with soft outstretched fluttering wings. This is what he has asked for every day and hour for eighteen years. This is what he pined after. Here it is--the summit, the end--the last page of the third volume. Good-bye, Colonel--God bless you, honest William!--Farewell, dear Amelia--Grow green again, tender little parasite, round the rugged old oak to which you cling!
Vanity Fair is sort of the British War and Peace, and part of the difference between the British and the Russian sensibilities is that Tolstoy is drawing on the style of historical writing, while Thackeray is drawing on the traditions of fiction; Tolstoy is creating a novel that's also a historical study, while Thackeray is creating a novel that's also a parody of other novels. This parody element is much more to the fore in the Amelia chapters, which is why they grabbed me more; the Becky story is a more standard rise-and-fall story, and a good one, but it always struck me as an adjunct to the Amelia story: Becky's selfishness is just a more open version of Amelia's selfishness, as both women take advantage of men who foolishly love them (Becky/Rawdon, Amelia/Dobbin) and want everyone around them to be focused on them, and only them. Becky's story is sort of the dark side of the traditional heroine, or what one of these selfish "heroines" would be without a privileged upbringing. But because her selfishness is more open, it's easier to know what to think of Becky: admire this quality, condemn that action. With the Amelia story, you find yourself hating characters for what you were raised to think of as good qualities, and liking characters for doing things that might seem mean and nasty in another novel. That kind of ambiguity is what makes the Amelia story so interesting to read. For me, anyway.
However, I think that any movie version definitely should play up the Becky story, as the Amelia story becomes dull when separated from the author's comments and that overlay of satire and parody of other novels.