You can use a feature of the MvvmLight called Messages. If your MainViewModel has a property RecentItems then you can listen for a message saying "Added to recent items" and respond to it with actually adding the item to the RecentItems collection. And also the message could be send by other ViewModel - for example DocuementViewModel.
You can my blog post about that, including an example project with messages.
Hope it helps.
the Telerik team
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