Does anyone know if vector tiles could carry adequate data to facilitate reverse geocoding (address lookup)? E.g. the polygon of a building containing its street address in its metadata. I’m surprised I haven’t seen that before, as reverse geocoding services can get expensive, though I wonder if it’s because I’ve misunderstood how vector tiles actually work.

Possibly, but why would you want to do this client side? This seems like the type of operation that makes much more sense in "OSM space", not vector graphics space.

Sure you could .... but to find something you would need to have the tile in question so you would need to calculate the tile to retrieve, and then find the object at the location (which is roughly equivalent to rendering the tile).

Doesn't seem to make sense when you can just run a nominatim or photon instance locally. Not to mention that currently you would typically have to add additional address data to the mix to get the quality of commercial geocoding services which makes the doing it via tiles even less attractive.

Yes, they could. You can attach whatever metadata you want to a vector tile feature.