The partnership plans to invest £534m in maintaining and improving existing homes and build more than 10,000 new homes over the next 10 years.
It aims to develop as many new social rented homes as it can.
The partnership claims it will save over £38m in its first 10 years, with development investment expected to support 400 new jobs.
Victory will retain its identity as a subsidiary of Flagship.
- Brexit has 'smashed property market sentiment to smithereens'
- £38m fund to provide more homes for vulnerable Londoners
- Planning granted for more than 700 rental Glasgow homes
Peter Hawes, chair of Flagship Group, said: “We are both strong and successful housing associations, but together we can be even stronger and more successful.
“Collectively, we have enhanced financial strength, greater effectiveness and efficiency, the ability to invest more in our existing homes, and support our customers.”
Philip Burton, vice-chair of Flagship Group, added: “This is an exciting time for Victory as together we can deliver more affordable homes and better-quality services to our residents and our communities.
“The partnership will create organisational growth and also greater opportunities for individual growth and progression for our staff.”