Pass value from one viewmodel to sub viewmodel

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WPF

MVVM

I'm using .Net FW 4.5, VS 2013 and SQLITE. I've got a "MainWindow" that contains tab items. Each tab item has it's own view and each view has its own VM. I am running a query on my MainWindow that returns data to a datagrid. Depending on which "SelectedItem" is selected I need my query to return relevant sub data to my tab which contains my view/viewmodel... how do I get my main VM to pass a value to my Sub-VM... I've tried "FindVisualParent" but it does not work. I think it's loading up my value after everything is loaded and therefore not applying my filer on my sub-VM query... Hope this makes sense.

Best methods applicable to MVVM would be appreciated.
(will try and upload sample project...)

on OneDrive... Download Sample Code