First of all: the character of Harriet Vane was introduced in Strong Poison as a means by which to rid Sayers of Wimsey, once & for all.
Catherine Kenney in The Remarkable Case of Dorothy L. Sayers:
In the essay “Gaudy Night,” DLS explains that she wrote Strong Poison “with the infanticidal intention of doing away with Peter, that is, of marrying him off and getting rid of him.” She says that she chose marriage rather than death as an end for her great detective’s career, because she did not want to risk being faced with Conan Doyle’s embarrassing predicament of killing a popular sleuth only to have to resurrect him later to appease outraged readers. This proved to be a practical concern; long after she ceased writing mysteries, readers continued to pester her for more Wimsey novels.
However! A glitch, known as “the problem of Harriet,” which is not unlike a problem called "Frankenstein": that is, a made creature who refuses to mind its maker, which is one of my very favorite problems in the world. From Dorothy L. Sayers herself, in an essay called—cleverly enough—”Gaudy Night,” as quoted in The Seven Deadly Sins in the Work of Dorothy L. Sayers:
I could not [at the end of Strong Poison] marry Peter off to the young woman he had…rescued from death and infamy, because I could find no form of words in which she could accept him without loss of self-respect…. [Several books later] I was still no further along with the problem of Harriet. She had been a human being from the start, and I had humanized Peter for her benefit; but the situation between them had become still more impossible on that account…. Her inferiority complex was making her steadily more brutal to him and his newly developed psychology was making him steadily more sensitive to her inhibitions…. At all costs, some device must be found for putting Harriet back on a footing of equality with her lover… I discovered that in Oxford I had the solution…. On the intellectual platform, alone of all others, Harriet could stand free and equal with Peter, since in that sphere she had never been false to her own standards. By choosing a plot that should exhibit intellectual integrity as the one great permanent value in an emotionally unstable world I should be saying the thing that, in a confused way, I had been wanting to say all my life. Finally, I should have found a universal theme which could be made integral both to the detective plot and to the “love-interest” which I had, somehow or other, to unite with it.
Thus that choice of stone? MISSION CRITICAL.
But! First there was another problem—that of Lord Peter himself, and of making him worthy of Harriet, which took Sayers five years and three additional books to resolve. Back to Catherine Kenney:
Rather than patching up their relationship [at the end of Strong Poison] and sending them off together in a contrived resolution, DLS did the artistically right but difficult thing. She resolved to do “radical surgery” on Peter,” to turn him into a whole human being from the bits and pieces of biographical and psychological data she had placed into even her earliest books. This decision shows that as an author, she had as much self-respect as she bequeathed to her characters. Her description of the tough-minded process of transforming Lord Peter is also rather comical, especially considering the number of people who have accused her of being in love with her fictional hero: “I laid him out firmly on the operating-table and chipped away at his internal mechanism through three longish books,” (presumably, Have His Carcase, Murder Must Advertise, and The Nine Tailors, unless she was counting Gaudy Night itself), in order to make him into a man an intelligent woman could accept.” In Gaudy Night, Sayers is obviously recalling this process in Harriet’s torment over “Wilfrid and Co.,” her new novel, which has “gone sticky” as a result of her trying for greater realism. To understand this gradual transformation of Wimsey, and of Sayers’s characters generally, into fully realized human beings, is to understand the development of her art and to see Gaudy Night as its apex.
I love that. She didn’t need to fix Harriet up for Peter; she needed to fix Peter for Harriet.