The Number Sixteen
I worry that having Vin make this connection is one of the more forced events in the book. She'd just finished telling everyone that she wasn't a scholar, and now she discovers a pattern of numbers hidden in the statistics of how people fall sick? My original intention for this was to have her be in a mind-set where she was looking for natural rules—because of her earlier discussion of Ruin and his rules—which then allowed her to see this pattern.
Rereading it, I'm not 100% pleased with it, but it's too late to make a change. I'd probably rewrite it so that Noorden or Elend make the connection, then let Vin connect that to what she's been thinking about. That would have been a much more natural progression.
Note that here, Vin misunderstands what these numbers mean. She's looking for rules that bind Ruin. What she finds is not that, but instead a clue left by Preservation. Numbers are understandable to people regardless of language, and so Preservation decided to leave some clues for people to discover that would hopefully lead them to follow the plans he'd set in motion. In my prewriting, I'd intended there to be more hard facts to be discovered in the workings of the universe—numbers hidden in mathematical statistics that said rational things, like the boiling point of water or the like. All as a means of Preservation hinting to humankind that there was a plan for them.
In the end, this didn't work out. I decided it would be overly complicated and that it would just be too technical to work in this particular novel. The only remnant of that plot arc became the number sixteen that Preservation embedded into the way the mistsickness works, intending it to give a clue about what the mists are doing to people. "You now are Allomancers!" is what this was supposed to scream. Unfortunately, the Lord Ruler's obfuscation of Allomancy—and the number of metals in it—left this clue to fall flat.