Please check with your local travel clinic and confirm whether you need to update your shots or not. We didnt want to take the chance as we were also travelling to Brazil so we updated our shots and received our yellow fever shot. There is also lots of info on the internet regarding shots recommended to visit these countries. Is it a worthwhile risk not to bother getting the shots?
Yes it is a reciperitory fee ( fancy name for visa but with less hassle), but I would certainly check on requirements to enter any country in SA. We had to get a visa and a yellow fever (and show proof) when we entered Brazil (being Canadian).
The falls are well worthwhile visiting, but really can be done with an overnite stay. Arrive one day, sleep overnite and spend the next day visiting the area and leave the next day. You can either do it on your own, or (unless they have changed) their are tours of differing lengths.
As the previous poster mentioned, The helicopter trip to view the falls is an excellent idea. We did that as well as walk around and it was certainly worth the cost. Check it out. Also that does seem like a long time to spend in BA - its a beautiful city but perhaps you could use that time in another place. Patagonia is certainly a beautiful option.