So Valentijn the Vampire fathered 79 babies in total: 6 humans, 1 spellcaster, 1 alien and 71 little vampires.
And I think everyone can already guess the main strategy: he turned many Sim women to vampires, before impregnating them.
He started with 'befriending' some pre-existing occult Sims, while he also worked a bit on his vampire lore skill, his vampire rank and his satisfaction points. The last one mostly to buy the shameless reward, so he was not ever bothered about getting it on with the next lady while the previous one was still around.
Clubs were used both to gather Sims and to persuade them into having Valentijn's babies. With the 'trait' requirement, he could make the club accessible for only occults (selecting alien, spellcaster, mermaid and vampire). He also started another club for human Sims. And if 'whoohoo' is a club activity, Sims need disturbingly little incentive to get it on. He tried for baby twice with each Sim, to increase the chance of pregnancy.
He stumbled into an alien night at the bar, which was also helpful. As soon as possible, he got the vampire perk and started turning Sims. He had a few human girlfriends while waiting for his victims to turn.
For a while, he had a routine of going into the towns to turn Sims, and then going home to start a club meeting, with the activities that that implies.
As he made his way through the list, he started travelling with the Sims he had transformed earlier. Travelling then triggered the complete transformation for them, and he had another rotation of club members at his fangs. He did need to ask them in person to join the club at the end, since there were too many vampires to find them all via the 'add club member' screen.
The lot traits used were convivial, romantic aura, and on a dark ley line. I probably should have used the on a ley line lot trait as well, although I'm not sure if that one works if the birth happens off lot.
I would like to give a shout out to Lily Feng, who had triplets despite disliking children. (Not even the first Baby Boomer challenge in which that happened for me.)
And thanks to the team. I really enjoyed playing this challenge.
(But it does feel a bit weird, now that I'm writing it down.)