In December, Rastignac receives the money he has asked for from his family, even though it comes at a great sacrifice for them. His increased spending attracts the attention of the other boarders, including Vautrin, who offers to mentor the young man.
He also proposes a plan: Vautrin wants to move to America and buy a plantation, but he lacks the money to do so. He suggests that he will arrange a lucrative marriage for Rastignac in exchange for a cut of the fortune. Rastignac is intrigued, and Vautrin explains his plan: Rastignac will court Victorine, and Vautrin will arrange to have a friend challenge Victorine's brother to a duel. After the brother is killed, her father will have no other heir and Victorine will become a wealthy woman, making Rastignac a partner in the wealth. Rastignac is initially disgusted by the plan, although Vautrin encourages him to take time to think about it.
Rastignac has learned from Goriot that the old man is not allowed to visit his daughters openly. However, he keeps track of them through their maids, and makes an effort to see them in public places.
One night, Rastignac accompanies Madame de Beauseant to the opera, where he sees Delphine de Nucingen for the first time. Rastignac by now has become much more cunning and charming, and makes a good first impression on Delphine by speaking well of her father, and implying that her sister is not a good daughter. He is invited to come and see her in the future, and when he returns to the boarding house, Rastignac confidently tells Goriot that he is in love with Delphine. Goriot is delighted by any news of his daughters, and encourages the relationship.
Despite his hopes about winning Delphine, Rastignac cannot help but be tempted by the money he could earn through Vautrin's scheme. He confides his moral dilemma in vague terms to his friend Bianchon, a medical student, who shrugs off the problem. Bianchon is more interested in the fact that he has seen the boarders Poiret and Madamoiselle Michonneau meeting with a man he recognizes as part of the police force. A short time later, Rastignac calls on Delphine, who is in a distressed mood. Surprisingly, she takes him to a gambling house, and sends him inside with a sum of money and instructions to win more. Naively, Rastignac is able to increase the money and returns to Delphine with the profits.
Delphine explains that her husband does not give her any money for her personal expenditures, and she had embarrassed herself by going into debt to her former lover. Now with the winnings, she can pay off the debt and be free. She is very grateful and tender with Rastignac, who leaves confident that he can make her his lover. When he returns to Goriot, the old man is very upset that his daughter did not come to him because she did not think he had any more money.
Although Rastignac is now moving in the high circles of Parisian society, Vautrin continues to insinuate that he will never be truly successful without a regular stream of income. Indeed, Rastignac does grow increasingly frustrated by his debts and by the fact that he has not yet slept with Delphine. By January, he is very close to agreeing to Vautrin's plan of wooing Victorine. He accepts money from Vautrin to pay off some urgent debts, but as soon as he has won more money gambling, he pays Vautrin back because he dos not want to be in debt to the man.
As it becomes apparent that Rastignac is starting to cultivate a new social persona, Vautrin steps forward as another mentor figure. His advice functions as a kind of exaggerated parody of what Madame de Beauseant has already told the young student. Vautrin is also of the mindset that anyone who wants to succeed in the world needs to be ruthless and totally self-interested.
Interestingly, his plan for how Rastignac can further his prospects also revolves around winning the affection of a woman. As a handsome man who lacks financial resources, Rastignac's good looks and charm are actually his best assets. The persistent advice that he should capitalize on these assets in order to advance his prospects inverts a typical gender dynamic where a beautiful young woman would use her looks and desirability to advance her position. However, Vautrin's scheme is both more shocking (since it involves a planned assassination) and more pragmatic (since it would ensure Rastignac has legal access to a fortune, not just recognition in society) than Madame de Beauseant's suggestion. Vautrin's criminal background makes him more willing to suggest radical action, and more attentive to ensuring confirmed financial gain.
Rastignac's reaction to the plan shows that he is torn between his moral principles and his ambitions. He is disgusted by the idea of being involved in the death of an innocent man, and also does not want to deceive the innocent and trusting Victorine. Vautrin's patience reveals his shrewd understanding of human nature: he knows that as Rastignac gets deeper into the world of Parisian society, he will crave money more and more. Vautrin's expectation proves accurate, since Rastignac rapidly becomes used to nice clothes, lavish entertainment, and gambling. Rather than finding his desires are satisfied with the social success he seems to be achieving, he starts to relentlessly crave more and more.
Rastignac's relationship with Delphine also follows this pattern. At first, his attitude towards wooing Goriot's younger daughter was mostly pragmatic:"Rastignac's perception that he can use Delphine to advance in society is at least as significant as any physical lust in his desire for her" (Berrong pg. 57). However, he quickly begins to feel genuine desire for her. This desire is intermixed with his overall desire for wealth and power, making the combination hard to resist.
Goriot also wholeheartedly supports the relationship between the young student and his married daughter. It seems that the elderly man is so desperate to cultivate a connection to his daughters that he sees a new lover as a way for him to also become closer to her. Goriot is also surprisingly liberal in his attitude towards his daughter's happiness. Because he believes Delphine's husband does not deserve her, Goriot would prefer her to be loved and doted upon by Rastignac. Goriot and Rastignac become drawn together by their mutual affection for Delphine, with Rastignac embodying the role of a surrogate son more and more. Some critics have even seen Goriot's intense and obsessive love for his daughters as being particularly aligned with his desire to select a new partner for Delphine; for example, James Walton writes that Goriot "at length plays the passive third in a ménage à trois, employing the young hero as Delphine's surrogate lover" (pg. 29).
The emerging details of Delphine's personal life offer a foreboding vision of what it might look like for Rastignac to obtain social success. Even though it seems like she has everything, Delphine is mostly unhappy and struggles with financial difficulties. She does not have a good relationship with her husband, and her previous lover has betrayed her. Because she has to maintain appearances and her standard of living, Delphine spends almost as much time worrying about money as Rastignac does. The presence of gambling in both their lives symbolizes the precarity and risk they are always living with.