public ICommand EmployeeSelectCommand
{
    get
    {
        return new RelayCommand((action) =>
        {
            if (PropertyChanged != null && action != null)
            {                        
                if (Service.OpenEmployeeDetailWindow(action as Employee))
                {
                    // Нажали сохранить - обновляемся
                    PropertyChanged(this, new PropertyChangedEventArgs("Employees"));
                }
            }
        });
    }
}

public class EmployeeViewModel
{
    private IDataContext Context;
    private IWindowService Service;

    public EmployeeViewModel()
    {
        Context = new DataContext(); // todo IoC resolve
        Service = new WindowService(); // todo IoC resolve
    }

    // Пачка дублирующего уныния, вместо пряморукого объекта Employee :(
    public int EmployeeId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public ICommand SaveCommand
    {
        get
        {
            return new RelayCommand((action) =>
            {
                var vm = action as EmployeeViewModel;
                Context.UpdateEmployee(vm.EmployeeId, vm.FirstName, vm.LastName);
                Service.CloseActiveWindow(true);
            });
        }
    }
}

<Window>
    <Grid>
        <TextBox Text="{Binding LastName}" HorizontalAlignment="Left" Height="23" Margin="10,10,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="300"/>
        <Button Command="{Binding SaveCommand}" CommandParameter="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}, Path=DataContext}" Content="Сохранить" HorizontalAlignment="Right" Margin="0,0,102,10" VerticalAlignment="Bottom" Width="75"/>
        <Button IsCancel="True" Content="Отмена" Margin="0,0,22,10" VerticalAlignment="Bottom" HorizontalAlignment="Right" Width="75" />
        <TextBox Text="{Binding FirstName}" HorizontalAlignment="Left" Height="23" Margin="10,38,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="300"/>
    </Grid>
</Window>