You can check out this approach demonstrating how an event from a UI Control can trigger a Command from your ViewModel. However, it is far away from MVVM to have a reference of your control in your ViewModels. ViewModels should not hold a UI controls / references to controls.
Typically, RadTreeView's and RadTreeViewItem's properties are bound to properties from your ViewModels. When you invoke a Command from your ViewModels, its execution can change some properties and they will reflect control's properties via the associated bindings.
All the best,
the Telerik team