Callum Paterson was the beneficiary of a poor Vestergaard backpass that condemned the Saints, who sacked Mark Hughes after last weekend's draw with Manchester United, to a seventh loss in 11 Premier League matches.

The visitor had looked to have steadied after a first half in which it rode its luck for long periods, Alex McCarthy making several important stops.

But there was little the goalkeeper could do to atone for his team-mate's game-changing mistake as ex-RB Leipzig boss Hasenhuttl was left to ponder how best to fix a defence that continues to undermine its bid to escape the drop zone. 

Cardiff was on top from the outset and would have been ahead early had Harry Arter connected cleanly with an inviting volley.

Mario Lemina was no more clinical in the opposite box, the midfielder slicing over when Paterson's defensive header from a corner looped into the air and rebounded kindly off the crossbar.

McCarthy, having twice foiled Josh Murphy, was called on once more before the break to tip over a thunderous Nathaniel Mendez-Laing drive.

Southampton improved after the restart and was aggrieved not to have been awarded a penalty on the hour, Victor Camarasa's outstretched leg appearing to make slight contact with Stuart Armstrong.

A contest short on attacking quality always seemed likely to be decided by a mistake and so it proved in the 74th minute, Vestergaard selling McCarthy short under scant pressure and enabling makeshift striker Paterson to seal his side's fourth home win in five fixtures.