Can I do this in code behind in a MVVM application?

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MVVM

Ribbon

I am relatively new with MVVM and there does not seem to be total consensus on whether some code is allowed in the .xaml.cs code behind file, or none at all. I will appreciate guidance with a specific problem:

I have a Ribbon application in the MVVM pattern. The ribbon has 6 tabs, each with its own set of commands and its own unique class in the ViewModel. Underneath the ribbon is a DataGrid that is bound to a different DataTable for each tab. The DataTable for each tab resides in its own class. This means the DataContext of the DataGrid must change each time the user clicks on a different tab. All of this works great, provided I change the DataContext in the window's code behind file. You cannot bind to the DataContext, so you cannot set the DataContext in the ViewModel, at least not as far as I could figure out.

Now my question: Is it permissible to set the DataGrid DataContext in code behind, or will the MVVM gods crucify me?