You must not vaccinate when you are sick. You cannot be cured of COVID-19 by vaccinating.
After acute illness (not COVID-19), especially flu-like symptoms, you should delay your vaccine until you are well.
If you are COVID-19 positive and asymptomatic, you should monitor yourself for 7 days for development of any symptoms, take enhanced precautions (mask wearing and social distancing) including avoiding attending settings where many people gather. If you are positive, you should wait at least 10 days before going for vaccination. It is not dangerous if you go sooner, however your vaccine response will be better if you wait a while.
After mild COVID-19 you can have your COVID-19 vaccine 30 days after recovering from the acute infection. This is usually around 40 days after your first symptoms.
After severe COVID-19 (needing oxygen) you need to wait 90 days for your vaccine.
Do not worry about missing your appointment, you can visit your nearest site when you are ready, book an appointment online https://vaccine.booking.health.gov.za/