<Button Command="{Binding AlertCommand}"></Button>

public RelayCommand AlertCommand { get; set; }

public void Alert()
{
   MessageBox.Show("message");
}

<Button PreviewMouseLeftButtonUp="OnMouseLeftButtonUp"></Button>

private void OnMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
     var ctx = (MainViewModel) this.DataContext;
     ctx.Alert();
}