One method is to put all of the data elements into a single file? Really? I've just counted up the number of columns there are in all of the data tables, in this application. There are 428 data columns that are a part of the old application. That's not including all of the lookup tables and their columns, The number will easily approach 600 columns from the various tables, that comprise this application. That would make for one really huge file, if everything is in it! So many columns all in one place, makes it difficult for more than 1 person to work on at a time. I don't know, that just seems wrong, to me, to put that much data in one place.
Who said anything about a single file?
Architect your application so that one instance of the business layer can be accessed from two different viewmodels. If the views are bound to the same instance, then there's no reason to pass data between views.
And remember, even though WPF shines with the MVVM pattern, it is not the only design pattern choice. This particular application may not benefit from separation of the view and the viewmodel.