Hi there and good to hear from you again. Pap tests are typically done following hysterectomy if the procedure was done for either cervical pre-cancers or cancers. It is essentially, as you mention, at that point a means of checking for vaginal cancer. There is seldom much risk for vaginal cancers, as that is a very, very rare disease, detected even less often than cervical cancer.
Read more at http://www.womenshealth.org/a/pap_test_after_hysterectomy.htm
. For more on vaginal cancers, go to http://www.cancer.org/docroot/CRI/content/CRI_2_4_1X_What_are_the_key_statistics_for_vaginal_cancer_55.asp?rnav=cri
(I love the American Cancer Society website, but their URL's are beasts!)