gistfile1.cs

public static void RegisterDepPropCallback(This DependencyObject owner, DependencyProperty property, EventHandler handler)
{
    var dpd = DependencyPropertyDescriptor.FromProperty(property, owner.GetType());
    dpd.AddValueChanged(owner, handler);
}

mainWindow.RegisterDepPropCallback(MainWindowClass.ViewModelProperty, (o,e) => {
    Console.WriteLine("ViewModel changed!");
});