Both sides were reduced to ten men before half-time, with Angola's goalkeeper Neblu shown an early red card for handling the ball outside his area and Namibia then losing Lubeni Haukongo to a second yellow card.
Dala netted either side of that Haukongo sending off, with those goals leaving Namibia with a mountain to climb in their first ever AFCON knockout match. A second-half comeback did not materialise, as Mabululu's finish sealed Angola's biggest ever AFCON win and ensured they will face either Nigeria or Cameroon in the last-eight.
It had looked like being so different when Angola went a man down after just 17 minutes, as Neblu charged off his line to save Bethuel Muzeu's lobbed effort, but did so having ventured outside the box.
A lovely move gave Angola the lead though, as Fredy and Gilberto combined brilliantly before the ball was squared to leave Dala a simple finish in the middle.
That marked the start of a disastrous four-minute spell for Namibia, who then had a man sent off themselves. Haukongo, already on a yellow card, tripped the heels of Mabululu as the forward made a run in behind, and a second booking swiftly followed.
From the resulting free-kick, Angola's lead was doubled, the ball whipped towards the far post and Dala rose highest to head in his fourth goal of the tournament.
Namibia never looked like forcing their way back into the match and the outcome was effectively sealed midway through the second-half, as Dala played the ball through to Mabululu and he found the far corner with a sensational curling effort.