One way to store the selected value in the View-Model, is to make a reference object in the View-Model and use the Selection Change Event from the View to change the View-Model's reference object.
this.GridView.SelectionChanged += new EventHandler<
void GridView_SelectionChanged(object sender, SelectionChangeEventArgs e)
((MyViewModel)this.DataContext).currentSelectedValue = ......
When I have a question about accessing the the V from the VM, I generally go back and see what I'm really trying to do. Alternatively you can make a Command, but to be honest, I haven't done that yet.