A Mzikayise Mashaba long-range thunderbolt was the difference between the sides in the end but, in truth, Vasco were the much better side over the 90 minutes.
The National First Division outfit gave their much-favoured Premiership opponents a good run for their money but were ultimately unable to make their dominance pay before Sundowns snuck the winner in the 60th minute.
By half-time, the Brazilians had not registered a single shot on target, with Vasco looking much more dangerous than their second-tier league status might suggest.
It was then the Cape side's turn to show what they could do going forward and Ndiviwe Mdabuka forced a good save out of Kennedy Mweene in the 13th minute.
Sundowns tried several times to attack down the left-hand flank but were repeatedly shut down by Gertse.
The Tshwane side's only shot on target before the goal came in the 31st minute, but Malajila's close-range effort was saved by Steven Hoffman.
Jonathan Armogam wasted a golden opportunity to put Vasco into the lead in the 37th minute when he latched on to good pass from Aleni Lebyane only to drag his shot wide.
After the break it was more of the same and anyone watching would have been excused for confusing which side was from the Premiership.
Just before the goal, Vasco spurned another glorious opportunity to take the lead when Mweene fumbled the ball in the box, but this time it was Armogam who couldn't find the target.
Brazilians fans will be concerned about the fate of Alje Schut, who was stretchered off the park in the 77th minute and will be hoping he will be back in time for the final.
After Schut's injury, new life was breathed in to Sundowns, who had two big chances to double their lead but were denied on both occasions by fine saves from Hoffman, although they had already done enough to progress to the tournament's showpiece event.
The Brazilians will take on Ajax Cape Town, who beat SuperSport United 2-1 on Saturday, in the final next month.