I'm biased Cherwell, as I have used VS on the South African run since 1997. I'm not in the airline biz and never have been but first went LHR ~ JNB in 1964 and have been fairly regular on it and in all classes at one time or another.
SAA, as I understand it having not used them on this run since (checks diary) 1986, are rather tired and behind the international curve of C or J.
BA, still good if they are actually flying. Their service in the air is still one I pay for. Last August I did a JNB return in WT+ (own money) and it was good. However, the VS oferring was slightly better.
VS are good. Yes, like any other carrier you can find plenty of irritated customers but the advantage of the non-stop to ZA is one that I'm always ready to pay for.