VS 450 then BA 590 at Heathrow
Johannesburg → London → Venezia
Timetable, London time
06:55VS 450 from Johannesburg at the gate
06:10BA 590 to Venezia leaves the gate
23 h 15 apart on the timetable, overnight
BA 590 leaves 23 h 15 after VS 450 arrives on the timetable: more than half a day, so not a connection we count.