I read because each reading has the ability to shift the angle of my worldview, and this is in turn feeds the will, the desire to live. This is my crack and as a result I am a ravenous and indiscriminate reader.
To start with, the language is great. I don't know how I have managed to survive without such phrases as the 'female inquisition' and 'twopenny gentility' in my vocabulary. Well, they are there now, thanks to Thackeray.
Another bonus is a good mixture of detached sarcasm and involved sentimentality, which is what the author aimed for himself (you'll find that out from the introduction, if you read those things). Which means that the story is funny, and when it is not funny, it has you fussing and worrying about the characters.
On the downside, there is some ancient didacticism (also alluded to in the introduction) which is a tad annoying. For example, Becky comes across as a very likeable character in the first scene (where she chucks the Johnson's Dictionary at a headmistress's assistant), when the narrator's tutting comes across as, well, square.
I know I will be hissed at for saying this, but Vanity Fair reminds me of Austen's writing. Now please remember that Austen, so badly sentimentalized nowadays, actually had a wicked sense of humour which was present in her novels. Detached sarcasm and involved sentimentality! Thackeray is (wants to be?) more on the sarcastic side, mind you.
And here goes the piecemeal review, as I am at work and need to get back to, uh, work.