can a settlement agreement be vacated or held invalid because my attorney failed to register with the court as my attorney of record?

Your attorney's appearance as attorney of record, in and of itself, should have no bearing on the validity or enforceability of your settlement agreement.  However, if you are claiming that your attorney had no authority to enter into such an agreement on your behalf (assuming that he entered into such agreement without your knowledge or approval) the fact that he never filed an appearance on your behalf could be evidence of that lack of authority, whereas if he had filed an appearance as your attorney, that would be strong evidence that he had, at least, "apparent authority" to settle the case on your behalf.