In most cases I would NOT suggest using any centralized place to share "events"/"notifications", like EventAggregator, etc.. This leads to later issues related with not clear relations between ViewModels. Such notifications makes sense in very specific cases when relations between listener/publisher is not known even on design stage.
I would suggest draw simple diagram with relations between ViewModels and find a way of using standard .NET events, so when you have clear realtionships between ViewModels like ViewModel1 has a reference to ViewModel2 so can subscribe to an event or provide own callback, so it will be easy to build such event notifications.
