Well, whether you are seronegative or not, the risk is the same. At this point you will have to weigh the risks or benefits of both the vaccine or the illness. How prevalent is yellow fever? How difficult is it to prevent contact with mosquitos? Will the time period you will be in SA coincide with less mosquito/yellow fever activity? Will you be out in the wild or staying urban? All of those factors must be weighed while you make your decision.

Good luck!