update a gtk.VBox dynamically

I have been using this website pretty often in order to solve small issues that I have while programming in Python. This time, somehow I could not find a suitable solution for my situation. So, here is my problem:

I want to dynamically add entries to a gtk.VBox widget. The problem is that it doesn't work the way I want it to work. I simply have a button, whose action is to add an additional widget to a VBox. Unfortunately the widget doesn't appear on the window. I guess, I have to add something like a repaint function call, but I didn't find anything like that. Here is a sample code, showing my problem:

import gtk

class DynamicVbox:

    def __init__(self):
        self.window = gtk.Window(gtk.WINDOW_TOPLEVEL)
        self.window.connect("destroy", self.close_application)
        self.window.set_size_request(400,320)
        #a hBox to put the button and the dynamic vBox
        hBox = gtk.HBox(False, 0)

        addButton = gtk.Button("add checkbox")
        addButton.connect("clicked", self.AddCheckButton)

        self.vBox = gtk.VBox(False, 0)
        self.vBox.pack_start(gtk.CheckButton("CheckButton"), True, True, 1)
        hBox.pack_start(self.vBox, True, True, 5)
        hBox.pack_end(addButton, False, False, 5)
        self.window.add(hBox)

        #start gtk
        self.window.show_all()
        gtk.main()

    def AddCheckButton(self, button):
        self.vBox.pack_start(gtk.CheckButton("CheckButton"), True, True, 1)
        print "adding checkbox..."

    def close_application(self, widget):
        gtk.main_quit()

 # run it

a = DynamicVbox()

A appreciate any help. Thanks in advance.

The new check button is there, but not visible until you call show() on it:

def AddCheckButton(self, button):
    button = gtk.CheckButton("CheckButton")
    self.vBox.pack_start(button, True, True, 1)
    button.show()
    print "adding checkbox..."