We have tried turning on vertex compression in Unity 5.4.0p3, and we cannot detect any difference on imported meshes or terrain (vert data looks same in PIX). Are we missing something?
(side note: I'd probably like to avoid this turning into a Unity support channel...)
There are several kinds of vertex compression in Unity.
One is compression setting in the mesh importer; this one saves storage (game data size) only. This is lossy, by storing less bits, but after loading the data is otherwise still in the usual format.
Another compression is the compression in player settings somewhere (the one where it can be set separately for each vertex component). And that one means different things on different platforms (e.g. some build platforms don't support some types of compression), and also depending on the mesh settings -- e.g. meshes that are marked "CPU readable" don't get compressed. Some meshes that are generated at runtime probably aren't compressed either, I guess Terrain patches would fall under that too.
If all that is not documented clearly, then it should! Please file a documentation bug with what is missing.