Since you need to access the view model from code-behind, you can use the DataContext of RadDataForm and set a property per your requirements.
//set property value
Can you please confirm if this fits your requirements?
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items