How to open a new window using MVVM Light Toolkit

Passing a message from ViewModel1 to View1 means to use the messaging capabilities in the MVVM Light Toolkit.

For example, your ViewModel1 could have a command called ShowView2Command, then it would send a message to display the view.

public class ViewModel1 : ViewModelBase
{
    public RelayCommand ShowView2Command { private set; get; }

    public ViewModel1() : base()
    {
        ShowView2Command = new RelayCommand(ShowView2CommandExecute);
    }

    public void ShowView2CommandExecute()
    {
        Messenger.Default.Send(new NotificationMessage("ShowView2"));
    }
}

View1 would register to receive messages in its code behind and display View2 when it receives the correct message.

public partial class View1 : UserControl
{
    public View1()
    {
        InitializeComponent();
        Messenger.Default.Register<NotificationMessage>(this, NotificationMessageReceived);
    }

    private void NotificationMessageReceived(NotificationMessage msg)
    {
        if (msg.Notification == "ShowView2")
        {
            var view2 = new view2();
            view2.Show();
        }
    }
}