It seems, like it's not possible to use a VBoxLayoutContainer within a TabPanel:
    public void onModuleLoad()
        TabPanel tab = new TabPanel();

        //ContentPanel f = new ContentPanel(); // --> this one works great
        VBoxLayoutContainer f = new VBoxLayoutContainer();
        f.add(new Label("Q"));
        tab.add(f, "bug");
        tab.add(new TextButton("testButton"), "showTestButton");

And that's, how it looks like:
The Label (or any other Widget) is rendered over the whole TabPanel. A ContentPanel doesn't make such problems, but since I am using VBoxLayoutContainer most of the time (automatic strech and flex on last item is just great), I hope this one will get fixed.