Making the unlikeable likeable

I am currently reading Vanity Fair out loud to my daughter. She is devotee of Rebecca and Gone With the Wind and so I thought I would introduce her to one of the first anti-heroines -- Thackery's Becky Sharp. (I also felt it best to read it to her -- at least in part -- so she can understand some more of the comedy)
In the hands of a lesser novelist, Becky Sharp would appear unsympathetic and without redeeming feature after all she is a social climbing selfish manipulative person. But Thackery contrives to engage the reader's sympathy.
In the first instance, he does this by introducing the vile Miss Pinkerton and her silly sister atthe start of the story. I don't know if the villianess in the Little Princess was based on this woman as perhaps such women are stock in trade. Miss Pinkerton has made a great fuss over the departure of Miss Amelia Sedley, and Thackery goes on about how she always gives her departing pupils a copy of Johnson's Dictionary. He also has a rather pompous letter from her to Miss Sedley's parents. However, when her sister brings a copy to be signed for Miss Rebecca Sharp, she refuses. A few pages later, when Miss Jemima hands Becky the Dictionary on the sly, the reader can feel a thrill as she tosses it back over the garden wall. Equally, Thackery exposes Miss Pinkerton's flaws by having Becky speak to her in French.
Thackery uses the character of Miss Pinkerton to create sympathy for Becky. She has attempted to grind Becky down and to use Becky as an unpaid music tacher as well as a French teacher. Because Miss Pinkerton has been odious, Becky Sharp gains the reader sympathy. It is very important in my view that Miss Pinkerton appears first and is horrid to her sister as it creates a certain impression in the reader's mind. If it had been done the other way around, I don't think the character of Becky Sharp would resonate as clearly. Say if the reader was introduced first to Becky packing and waiting for the coach that will take her away from all this. It is the fact that Thackery creates a caricature of an over bearing, self-important headmistress first that gives Becky her punch as it were. In the first chapter, Becky appears as a minor character, rather than the major character and it works.
It is certainly a lesson to me -- when creating a potentially unlikeable heroine, you need to put her in a position where the reader will be rooting for her from the outset, rather than letting the reader discover her good qualities as the book goes on.
As Vanity Fair goes on, Becky will become more manipulative, scheming and in general less likeable but because the reader's first impression of her is someone striving against the odds, the reader retains a certain amount of sympathy. You can understand why she is acting this way, why she feels she has no alternative.