C# code:

// view code-behind
    public partial class MainWindow : Window
    {
        private MainWindowViewModel MWVM;
        public MainWindow()
        {
            MWVM = new MainWindowViewModel();
            DataContext = MWVM;

            //////////////////////////////////
            MWVM.DataUpdated = OnDataUpdated;
            //////////////////////////////////

            InitializeComponent();
        }
        
        //////////////////////////////
        private void OnDataUpdated()
        {
            MessageBox.Show("fart");
        }
        //////////////////////////////
    }

// view model
    class MainWindowViewModel : ViewModelBase // ViewModelBase implements INotifyParameterChanged and IDisposable if you give a poo poo
    {
        public Action DataUpdated { get; set; }
        
        // blah blah blah

        private void UpdateData() // pretend some RelayCommand bound in the view calls this :effort:
        {
            // blah blah blah

            ///////////////////////////////////
            if (DataUpdated != null)
                DataUpdated();
            ///////////////////////////////////
        }
    }