When I logged into the AARP site I used the link to tha BA Portal that showed the it was the discounted BA site in the upper right corner. When I compared the regular BA.com pricing with what I booked through the portal it comes up with a $130 discount.
As far as booking this, I used the Multi-City search and changed the departure country to Canada and Vancouver was then available in the drop down pick-list. I used YVR-JNB then JNB-CPT, CPT-JNB and finally JNB-ORD. Came out several hundred dollars lower than booking a straight ORD-CPT roundtrip.