Green’s Dictionary of Slang

(S.Afr. township) a ‘live-in lover’, a common-law partner, an unsolemnized marriage; also as adj.

1959		L. Longmore Dispossessed 32: Inevitably comes the day when he fancies someone else and tries to get rid of his vat en sit woman.
1962		Casey ‘Kid’ Motsisi ‘Kid Booze’ Casey and Co. (1978) 47: The guy who decides to marry his ‘vat en sit’ girl after a donkey’s age.
1977		J. Sikakane Window on Soweto 17: There had developed a system, dubbed ‘vat en sit’, meaning practically ‘let’s marry on our own without conforming to family tradition and the church.’.