If my parents had the smallpox vaccination, are they still immune 30-40 years later?

Agree w Dr. Raff. A study called “Duration of antiviral immunity after smallpox vaccination.” Hammarlund E. etal (Nat Med 2003) showed that >90 % of volunteers vaccinated 25 to 75 years prior still had significant humoral / cellular immunity. ** http://www. Ncbi. Nlm. Nih. Gov/pubmed/12925846**
Yes. The vaccination is with a live cowpox virus and for most individuals provides lifelong lasting immunity.