If you hold a South African passport then yes, you need to take an  IELTS exam (within 12 months of travel) to be eligible for a work permit as a TEFL teacher in Vietnam.
You also have the option of taking the Vstep exam in Vietnam although it generally takes much longer to obtain the test results which will slow down processing your work permit.
We recommend sitting the IELTS exam before traveling to Vietnam as this means that you will not need to legalize the test results.