When you close the dialog in the following example i get an error message
in the console:

[sumpan@e dialog_entry_test]$ ./dialog_entry_test 

(dialog_entry_test:17708): Gtk-CRITICAL **: file gtkcontainer.c: line 878
(gtk_container_remove): assertion `GTK_IS_CONTAINER (container)' failed


Is there a problem placing a Gtk::Box inside another Gtk::Box? If you
change the entry to a pointer and use manage to initialize it everything
works as expected....

---
#include <gtkmm.h>

using namespace SigC;

class TestDialog : public Gtk::Dialog
{
public:
  TestDialog() : Gtk::Dialog("Test-dialog", true, true) {
    get_vbox()->pack_start(vbox);
    vbox.pack_start(e);
    show_all();
  }
private:
  Gtk::Entry e;
  Gtk::VBox vbox;
};

int main (int argc, char *argv[])
{
  Gtk::Main kit(argc, argv);
  TestDialog d;
  kit.run(d);
  return 0;
}