In a good MVVM design, you will be able to detect the current RadTreeViewItem (the selected one), current Row in a GridView, current item that is in edit Mode and etc. So I suggest you work with ViewModels only - bind the controls in the Template to a properties in your ViewModels and change only the ViewModels.
However, if you need to access a control down or up in the Visual tree you can use the Telerik's extension methods ParentOfType<>, or ChildrenOfType<>. Two things are important here- Popups( the DropDownContent of a DropDownButton is usually a Popup) are in a different Visual Trees than the other controls so ParentOfType often cannot be used with them. On the other hand, these two methods are potentially slow - so you better avoid them.
the Telerik team
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