Just a few points to add: arrivals and departures are on different floors, so you'll need to go up/down an escalator each way. The domestic security is at the far end of the domestic terminal, so keep walking past the checkin desks; the SAA lounge is upstairs.

If you are more than 3 hours early for the LX flight at JNB there is an early checkin desk for F/J customers... and as the VS CH only opens 3 hours before the LX flight you should be able to get an additional invitation for a different lounge which you can use before the CH opens.

The VS Clubhouse at JNB is one of my favourite lounges: excellent showers, great food menu and home of the magically refilling beer!

BC