In order to add your own connectors using MVVM you will need to create an Attached property to the RadDiagramShape and bind its Value to a collection from your ViewModel. In the attached property you have direct access to the shape and its Connectors collection and you could add the new connections to the collection. Also you could choose to either ignore or use the default five connectors using the UseDefaultConnectors property of the shape.
I have attached a sample project for your convenience demonstrating the approach. Please let us know if you need further assistance.
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