WPF MVVM: Binding a different ViewModel to each TabItem?

You can indeed add the view models for your tabs to a main view model. You can then bind to the child view models in the XAML for your tabs.

Say that you have three viewmodels: MainViewModel, Tab1ViewModel, and Tab2ViewModel. On your MainViewModel you keep a collection of your tab viewmodels:

class MainViewModel
{
    ObservableCollection<object> _children;

    public MainViewModel()
    {
        _children = new ObservableCollection<object>();
        _children.Add(new Tab1ViewModel());
        _children.Add(new Tab2ViewModel());
    }

    public ObservableCollection<object> Children { get { return _children; } }
}

After setting the DataContext of your main window to your MainViewModel you can bind the DataContext of your tabs by referencing the Children property:

<TabControl>
    <TabItem DataContext="{Binding Children[0]}" x:Name="Tab1" Header="Tab1" >
      <!-- Tab content -->
    </TabItem>
    <TabItem DataContext="{Binding Children[1]}" x:Name="Tab2" Header="Tab2" >
      <!-- Tab content -->
    </TabItem>
</TabControl>

class MainViewModel
{
    ObservableCollection<object> _children;

    public MainViewModel()
    {
        _children = new ObservableCollection<object>();
        _children.Add(new Tab1ViewModel());
        _children.Add(new Tab2ViewModel());
    }

    public ObservableCollection<object> Children { get { return _children; } }
}

Now in XAML bind the Children to ItemsSource. It will generate each Tab for every viewmodel we have added into the observable collection

    <TabControl ItemsSource="{Binding Children}"/>