As mentioned in a previous reply, you should search for the other discussion for the same issue. There, you would learn, that the circular reference comes from portal_catalog. You do not see it in your output because your items are in fact so called "catalog brains"; they have a specialized repr which does not show the hidden catalog reference.
At the moment, I suppose that plone.app. content.vocabulary does not handle the "catalog brain" case correctly: it should cut of the portal_catalog reference when it constructs its "item" from a "brain".