public Binding ExpandStateBinding { get; set; }

Public Property ExpandStateBinding As Binding

<dxg:TreeViewControl ItemsSource="{Binding EmployeeDepartments}" 
                     ChildNodesPath="Employees" 
                     TreeViewFieldName="Name"
                     ExpandStateBinding="{Binding Expanded}"/>

public class EmployeeDepartment {
    public string Name { get; set; }
    public ObservableCollection<Employee> Employees { get; }
    public bool Expanded { get; set; }
}

Public Class EmployeeDepartment
    Public Property Name As String
    Public ReadOnly Property Employees As ObservableCollection(Of Employee)
    Public Property Expanded As Boolean
End Class