Hey @sparrow thanks for doing some digging!
We're actually using the Sphinx search engine for the search here. There was a semi-working plugin for Vanilla that I used at the time, and had to mess with to get it working.
The main reason for using Sphinx over Vanilla's built-in search was because Vanilla's search (at the time) was even worse than Sphinx.
Also, the forums do need to be upgraded to a more recent version of Vanilla, but I just haven't had the time or head-space to do it. I'm also not totally in love with Vanilla, but I don't particularly feel like trying to migrate all our posts to another forum software (which again takes time).
But if you'd like to spin up Vanilla, and see what you can do, please feel free to!
I recently deployed Flarum forums, which seem kinda nice, and were easier to get up and running in Docker than Vanilla when I last used it, so that may be an option too, if we can figure out how to migrate the posts.