There is only 1 occasion of reported speech from the partner, and he would have used ‘tu’ when addressing her. 

The subsequent “you” is the author again, and addressing women generally, hence “vous” is used. 

Thank you Maarten for your response. When I worked on this exercise the second time, I read the Hint about addressing the audience. So I agree with you that "vous" should be used in this case.

"vous" is to address the reader, not the partner.