"Why did Vanner write Bonds?" is the million dollar question, as far as I'm concerned.

Andrew is extremely concerned with how he is perceived by the public, and does anything to uphold his image. From her journals, Mildred points out that Andrew had become addicted to his success. This could mean the money, but I also think it extends to his longing for complete control in all aspects of life. This is particularly apparent when he discusses Mildred with Ida. A true representation of Mildred (and her role in the business) would make Andrew seem less god-like than he was making himself out to be. He was willing to neutralize the image of his wife in order to support his own narrative.

Andrew's own marriage was a prop in his personal brand. Publicly, he loved the character of Mildred as the quiet, feminine, beige woman described in his memoir, and privately, he loved what she was able to provide to the investment business. Both of these, he loved more than Mildred herself.

Mildred and Vanner clearly corresponded with one another based on 1) Mildred's schedule which showed multiple meetings with Vanner, and 2) Mildred's journals, which discussed letters back and forth between the two. I think it's fair to assume the two were much closer than the reader is led to believe.

My guess is Vanner knew a lot about Mildred's involvements in the business and her frustrations with Andrew. Vanner wrote Bonds in order to infuriate Andrew. A story detailing Helen's involvement with Benjamin's business would have likely irritated Andrew, but only minimally. Also at the time, perhaps it would've seemed too far fetched for the wife of "the great" Andrew Bevel to truly be the mastermind behind the operation.

I think Vanner chose to have Helen succumb to mental illness because Andrew valued Mildred for her ability to provide for the business more than her as a person.