The woman repeatedly reported Craig Hutsby, of Eastwood, Nottinghamshire, for threatening behaviour and breach of bail conditions in July 2014. The following month Hutsby raped her.
The IPCC report revealed the attack could have been prevented had arrest forms been filled out immediately.
Det Supt Rachel Walker said officers had attended misconduct meetings.
The Independent Police Complaints Commission (IPCC) concluded that PC Sofi Essex has a case to answer for misconduct for not creating an "arrest package" - paperwork completed before arrest attempts can begin - when asked.
"Although eventually completed, the delay resulted in no action being taken to locate and arrest Craig Hutsby prior to the rape being committed," it said.
PC Joseph Lovelock has a case answer for misconduct for failing to properly complete a form for the victim, the IPCC concluded.
"This meant she was not identified as being at high risk of harm and appropriate measures to manage that risk could not be made," it said.
Action taken by Derbyshire Constabulary
The investigation also highlighted unsatisfactory performance by two police call handlers, who did not properly communicate Hutsby's bail conditions to officers investigating the woman's reports.
IPCC Commissioner, Derrick Campbell, said: "This investigation has highlighted failings by Derbyshire Constabulary to protect a vulnerable woman who went on tragically to become a victim of rape."
Det Supt Walker, said: "We agree with the IPCC that the level of service given by the officers and call handlers was not of the standard we expect.
"We have already taken action to improve the service we provide."
Hutsby was convicted of rape in February 2015 and sentenced to nine years and five months.
He was also served with a Protection from Harassment Order and will be on the Sexual Offender Register for life.
A man raped a woman before being arrested due to "failings" by Derbyshire Police, a report has found.