Activating An Existing DockingWindow (MVVM)

I'm implementing a Docking/MDI application with the MVVM pattern.  Whenever a new DocumentItemViewModel is added to the DockSite's DocumentItemsSource, a new tab opens - easy peasy.

However, I'd like to prevent the same document (file) from being opened in duplicate tabs.  So when the user selects a file, I compare its path+filename to each existing DocumentItem, and if it already exists, I don't open it.  So far so good.

Next, I'd like to activate the existing file's tab when the user selects it.  I can think of two solutions to this:

1) Give each DocumentItemViewModel a reference to its DockingWindow, which would be "injected" in OnDockSiteWindowRegistered (DockSiteViewModelBehavior.cs in the sample code).  That way, once I've determined that a given DocumentItemViewModel is a duplicate (aka is already opened), I can call Activate() on its DockingWindow.

However, while this approach should work, it seems to me that giving the DocumentItemViewModel a reference to its DockingWindow somewhat breaks the View-ViewModel separation in MVVM.  Or would such a reference be considered OK?

2) Give the main window's ViewModel a reference to its DockSite, and when the user selects a file, check it against each of the DockSite's DockingWindows (rather than checking each DocumentItem).

However, this seems to suffer from the same issue: now I'm giving the main window's ViewModel a reference to a "view" object.

I'm pretty new to the whole MVVM thing, so perhaps the above are perfectly acceptable.  But if not, what might be a "correct" approach to activate an existing tab when the user re-selects its file - without breaking the MVVM separation?