The plot of Vanity Fair primarily follows the lives of two women: Becky Sharp, a girl who is born poor and strives to gain money through cunning and social standing through marriage; and Amelia Sedley, a kind, simple girl who is born into money and engaged to the vain Captain George Osborne (though she is loved by the loyal Colonel Dobbin). During the novel, both women enter loveless marriages and are beset by sudden financial ruin (largely as the result of bad luck and disinheritance).
The novel engages with themes of deception and manipulation, as characters attempt to get what they want (often in the face of drastic consequences) by using other characters, either through marriage, business deals, or other means.
The novel also considers themes of desperation. Vanity Fair presents the reader with the question of what kind of behavior desperation justifies. It also asks whether characters do gain happiness and security (or should gain them) by being selfless and good, or by acting in their own interests.
The novel also deals with themes of loyalty. Becky Sharp is extremely disloyal, as is George Osborne. Colonel Dobbin, by contrast, is a paragon of loyalty.
One of the most prevalent themes in Vanity Fair is, as the title suggests, vanity. The characters are primarily worried about their own vanity and social position more than anything else. The characters are willing to do anything, lie, steal, cheat, etc in order to advance their social status, all in the name of vanity. Becky Sharp is probably the most vain of everyone. She is so proud of her good looks and her clever wit, and has quite an ego. She uses her husbands for advancement, not really caring about them personally. Her first husband is just a stepping stone to get her into different social circles. When she marries her second husband, she convinces him to take out a life insurance policy, and he dies very shortly after, from being poisoned. Becky is not above doing whatever it takes to satisfy her own vanity.