Confusing Question About MVVM and Window Closing Event (Setting Cancel to True Works Once)

Strictly adhering to MVVM, I'd like to cancel the closing of a window when the user clicks the "X" or standard Windows close button.

The ViewModel is as follows:

private RelayCommand<CancelEventArgs> _windowClose = null;
public RelayCommand<CancelEventArgs> WindowClose {
	get {
		if (_windowClose == null) {
			_windowClose = new RelayCommand<CancelEventArgs>(
				(args) => {
					args.Cancel = true;
					BeforeExitViewModel tmp = new BeforeExitViewModel();
					WindowManager.ShowWindow(tmp);
				}, (args) => { return (true); });
		}
		return (_windowClose);
	}
}

I pop-up a quick and dirty ViewModel so the user can save changes if they are dirty or cancel the closing event (keep their work open). From the view:

<i:Interaction.Triggers><i:EventTrigger EventName="Closing"><nvis:EventToCommand Command="{Binding WindowClose}" /></i:EventTrigger></i:Interaction.Triggers>

I'm new-ish to MVVM so this really seems correct and simple to me. Trouble is, it works... ONCE!  If the user clicks "Close" then cancels, the window view remains open until they click a second time. Although the RelayCommand DOES fire, it seems setting e.Cancel to true has NO EFFECT.  Why is this?

I know there are workarounds and I can use code behind, but I wanted to post on here to learn from this issue.  Thank you in advance for any helpful replies I get!