Load UserControl in TabItem

Thanks Rachel for your answer. But it enforces declaring the DataContext during compile time itself. Like you did, relating each of the Views to their respective ViewModels in the DataTemplate of TabControl. We can achieve dynamic View-ViewModel linking when move this out to ViewModel. Here's how:

XAML:

<TabControl.ItemContainerStyle>
      <Style TargetType="TabItem">
          <Setter Property="Header" Value="{Binding Header}" />
          <Setter Property="Content" Value="{Binding Content}" />
      </Style>
</TabControl.ItemContainerStyle>

VM:

 public ObservableCollection<TabItem> TabItems { get; set; }
 public MainWindowViewModel()
        {
            TabItems = new ObservableCollection<TabItem>
            {
                new TabItem{Content = new TabAView() {DataContext = new TabAViewModel()}, Header = "Tab A"},
                new TabItem{Content = new TabBView(), Header = "Tab B"}
            };
        }

We can even make use of Action delegates to delay and invoke initialization of the TabItems only upon Tab SelectionChangedEvent. This achieves lot of memory saving if the UserControl Views have many UI elements.

Ideally, the TabControl.ItemsSource should be set to a collection of ViewModels, and DataTemplates should be used to tell the WPF to draw each ViewModel with a specific UserControl.

This keeps between your business logic (ViewModels) completely separate from your UI (Views)

For example,

<TabControl x:Name="MyTabControl"
            ItemsSource="{Binding TabViewModels}"
            SelectedItem="{Binding SelectedTabViewModel}">

    <TabControl.Resources>
        <DataTemplate DataType="{x:Type my:ViewModelA}">
            <my:ViewAUserControl />
        </DataTemplate>
        <DataTemplate DataType="{x:Type my:ViewModelB}">
            <my:ViewBUserControl />
        </DataTemplate>
        <DataTemplate DataType="{x:Type my:ViewModelC}">
            <my:ViewCUserControl />
        </DataTemplate>
    </TabControl.Resources>

    <TabControl.ItemContainerStyle>
        <Style TargetType="TabItem">
            <Setter Property="Header" Value="{Binding Header}" />
        </Style>
    </TabControl.ItemContainerStyle>

</TabControl>

ViewModel containing TabControl's DataContext:

TabViewModels = new ObservableCollection<ITabViewModel>();
TabViewModels.Add(new ViewModelA { Header = "Tab A" });
TabViewModels.Add(new ViewModelB { Header = "Tab B" });
TabViewModels.Add(new ViewModelC { Header = "Tab C" });

SelectedTabViewModel = TabViewModels[0];