The authorities have been enforcing the policy and have sent a number of people who do not have the required (genuine) YF card packing - and I, for one, would hope that they enforce the policy strictly as long as they feel there is a danger of that disease being introduced into South Africa. The authorities have no idea where you might have been while in Zambia or with what or whom you may have come in contact.
Nutrax has given the possible actions that might be taken against anyone who does not have the required certificate - as follows:
+" If you don't have a certificate, you will be refused entry.
If you have a certificate, but it is not yet valid, because it hasn't been 10 days since you got the shot, you will be quarantined until the cert. is valid or until 6 days have passed.
If you have a medical exemption, you will be
> allowed entry
> required to report any fever or other symptoms to the health authorities
> be placed under surveillance. "+
You might be lucky and slip through the net, but I would not bank on it.
Victria Falls Town and Livingstone are some 15 km apart - not parts of the same town.
If you have the time you could get your YF in Johannesburg before departing to Livingstone, else cut your losses and fly to Vic Falls instead of to Zambia. You would see by far the best part of the falls, while sacrificing the view from the Zambian side. See http://www.airpano.ru:80/files/Victoria-Falls-Zambia-Zimbabwe/start_e.html
Enjoy.
Rudi