A common pattern for this style of problem is MediatorMediator (a class that both view models reference and can be used to pass messages between the two).
The Mediator class has since been moved to the Cinch WPF/SL MVVM FrameworkCinch WPF/SL MVVM Framework, which appears to still be actively developed/supported.
The pattern I prefer is the Event AggregatorEvent Aggregator, an example can be found in the Prism framework. In this pattern different view models subscribe to events from the aggregator and others publish events.
Hope this helps