Buttler and Bairstow also struck fifties as the visitors chased down 223 to clinch a 2-1 series victory.
England win thriller to square series as South Africa finished with 202 in a chase of 205 in the second T20I.
England stuttered in the 178-run chase, losing five wickets for 43 runs.
England win by two wickets in Johannesburg to avoid first series defeat since 2017.
Dale Steyn returns as South Africa announce squad for T20I series against England.
Hosts maintain 1-0 series lead as 2nd ODI gets abandoned because of rain.
New skipper Quinton de Kock Stars and Temba Bavuma as South Africa trump England in 1st ODI at Newlands.
England complete victory over South Africa - and a 3-1 series win - with a day to spare in the final Test in Johannesburg.
England set weary South Africa world record 466 to win final Test
Mark Wood shines with bat and ball to put South Africa on the brink. Nortje bagged his maiden fifer