Africa - Connecting flights and checked baggage

I'm flying from Newark to London and then to Johannesburg on Virgin Atlantic. I'm then flying from Johannesburg to Lusaka, Zambia on South Africa Airways. I know I will have to pick up my luggage and then check it again when I arrive in Johannesburg. My question is what if my luggage is delayed and I don't get to pick it up before I depart for Zambia? How will it get to the Lusaka airport?

First just check if VS and SA dont have an interline agreement and will allow the luggage to be checked all the way to Zambia. They do have some code share agreements (IIRC), so it may well be that this is also possible. In that case you have an Int-Int connection in JNB and you also avoid immigration, which is a definite bonus!
If not, what is your connection time in JNB? It does not seem very likely that you arrive in time for the connection and somehow your luggage does not, unless it is misdirected at LHR (which is of course always possible but in my experience not very likely). Is the connection at LHR very tight?

I have an 11 hour layover at LHR and a 7 hour layover at JNB. I guess it's not likely, I don't know. What causes lost/delayed baggage in the first place?

I have an 11 hour layover at LHR and a 7 hour layover at JNB. I guess it's not likely, I don't know. What causes lost/delayed baggage in the first place?

Those are looooong layovers, there is no way your luggage will suffer a 'short connection problem'. But there always is a small chance of some error causing a bag to go astray. I have had that on 2 or 3 occasions in my travelling life and the bag has always shown up a day later. Normal practise with SAA is that they deliver it to you at your house (in my case in CPT) or your hotel. But I have also heard accounts with other airlines where you'll have to come to the airport to collect it.
The 7 hour layover in JNB is a potential hazard because of the amount of 'interference' with luggage there. The safest (in my opinion) would be to claim the bag and only check it in less than 2 hours before the next flight. But then you cant try interlining as I suggested in my earlier post and you sit with your bag landside for a few hours which is far from optimal. What were you planning to do in those 7 hours? There is a luggage deposit in JNB and as far as I understand it's safe. I dont travel through JNB a lot, maybe others on this board can give suggestions.