Как инициализировать или связать ListItems, которые определены в App.config, в поле со списком VB.Net wpf

Как насчет совсем другого подхода. Сохраните список в формате XML из отдельной утилиты. Затем десериализуйте и привяжите к ComboBox.

Class MainWindow
    Private lst As New List(Of String)
Private Sub btnFillCombo_Click(sender As Object, e As RoutedEventArgs) Handles btnFillCombo.Click
    '1. Create an instance of XML Serializer
    Dim xs As XmlSerializer = New XmlSerializer(GetType(List(Of String)))
    '2. Create a Stream for your file
    Using fs As Stream = New FileStream("MyList.xml", FileMode.Open, FileAccess.Read, FileShare.None)
        '3. Deserialize your Stream and cast to List(Of String)
        lst = CType(xs.Deserialize(fs), List(Of String))
    End Using
    cboList.ItemsSource = lst
End Sub


    Private Sub btnSaveList_Click(sender As Object, e As RoutedEventArgs) Handles btnSaveList.Click
        ' FileStream(FileName, Mode, Access, Share) intellisense overload #9 out of 15
        Using fs As Stream = New FileStream("MyList.xml", FileMode.Create, FileAccess.Write, FileShare.None)
            Dim xs As XmlSerializer = New XmlSerializer(GetType(List(Of String)))
            xs.Serialize(fs, lst)
        End Using
    End Sub

    Private Sub btnLoadList_Click(sender As Object, e As RoutedEventArgs) Handles btnLoadList.Click
        lst.Add("Mathew")
        lst.Add("Mark")
        lst.Add("Luke")
        lst.Add("John")
    End Sub
End Class

btnLoadList и btnSaveList будут находиться в отдельной утилите, запускаемой только один раз.