The novel is set in the early 1800s, about twenty years before it was written. This time disparity is actually a much bigger deal than it sounds. Why? Well, Thackeray is writing in 1846. Queen Victoria has been on the throne for about 10 years, and the country has taken a decided and rather hard turn toward the conservative, the prim, the proper, and all those other stuffy and fussy adjectives we associate with Victorianism.
However, the period Thackeray is writing about, 1810-1820 (Napoleon's loss at Waterloo was in 1815), is way before this tightening and straightening of morals and standards. During this time George IV rules England as Prince Regent (which is why the period is called the Regency). He is a spoiled, decadent guy who loves to live large and laze in the lap of luxury. (His lifestyle is brought to us by the letter L.) Society is still lingering in the easygoing moral approach of the 18th century, which was a less rigid, more open, less inhibited kind of time.
So what happens when you get an author writing about relaxed morals for a totally repressed audience? He starts to gloss over things and to use euphemisms when talking about sex (for instance, Rawdon's conquests before he meets Becky) and to express disapproval about qualities that might have just gotten a pass before (for example, Becky's lack of maternal instincts).
Doesn't some of this novel read like a travel guide? Thackeray's narrator is constantly busting out with asides like "Check out the wonders of Brussels, where there is lovely shopping to be had if you've brought money!" or "Ah, Pumpernickel – the jewel of this corner of Germany, where there are fun things to do and charming people to do them with!" Partly this is because, as Thackeray himself admitted, the easiest way to get the plot to go forward was to send his characters out of the country and into new and different settings where they would be forced to mingle and interact in new and unprecedented ways.
What's most fun about these trips abroad is the very strict application of the Vegas rule – you know, What Happens on the Continent Stays on the Continent. ("The Continent," by the way, is mainland Europe.) Check out how the uber-snobby Bareacres family is happy to hang out with George in Brussels (well, they're happy to spend his money for him), fully expecting to not even admit that he exists when they get back to England. Jos's experience is different but falls under the same social law. In Brussels, he is...well, less than brave. A lot less. In fact, he is the only one who runs away from the supposed approach of Napoleon. Yet, after they leave the city, the story he ends up with is the one that gets him nicknamed Waterloo Sedley and puts him almost into Wellington's tent during the battle. And, of course, on a morbid, but literal note – George dies in Belgium and there his body remains.
How is this different from the way London and its countryside work? When the gang is in England, there don't seem to be any geographic boundaries in the way information travels. Mrs. Bute knows all there is to know about Queen's Crawley, down to the amount of hops they put in their beer. Mr. Osborne knows all about Mr. Sedley's failing transactions on the stock exchange, down to which ships Sedley's cargo is on. Miss Crawley in no time at all finds out not only Becky's entire history, but that of her suspiciously immoral parents. What makes Becky really amazing is that she is somehow able to keep the merchants that supply her house in London in the dark about her financial situation – a totally uncharacteristic ability to be secretive in a place where there don't seem to be any secrets.
There are many good ways to think about the differences between Amelia and Becky. Which makes sense, since we are obviously meant to compare and contrast their looks, their personalities, their childhoods, their attitudes towards the world, and so on. But let us suggest yet another way in which they are fundamentally different. The shy and retiring Amelia is all about private life, while society-hungry Becky thrives in the public eye.
Amelia wants nothing more than to sit at home, first with George, then with her son, and finally with Dobbin. This is a woman who wouldn't ever leave the house if she didn't have to. She's at her best indoors, preferably in her own space. There she can be charming and pretty, and she can even rise to the occasion every now and again (like when she tends to the wounded soldiers in her hotel room in Brussels). Every time we see her out in the world, she is totally lost, looks awful, and generally can't cope (at the ball in Brussels, or walking around London trying to sell her little paintings, for example).
Becky is the opposite. She lives for being seen and thrives on public life. She is at her very best when she's out and about (riding into Brussels amid a throng of generals, being the toast of Paris society, wowing everyone with her performance at Lord Steyne's charades party). She is totally unable to deal with domestic life, most notably in her total lack of desire to be a mother or deal with her son in any way.