Well, it's still a bit buggy thanks to the addon not having been updated for the latest version of vBulletin yet, but the mentions system is back and working.
If you want to mention someone in a post, there's two ways you can do it.
1. Manually write in the BB code. For example, to mention me you'd write (removing the extra spaces)
[ mention ] Archaic [ / mention ]
2. Place an @ at the start of their username, and an ; at the end of their username. For example (removing the space at the end)
The old system of just putting an @ before a username with nothing at the end will not work. This is due to a bug in the addon in the latest vBulletin. There's nothing that we personally can do about that but wait for the addon to be updated, so please don't ask us when that will be re-enabled, because we simply don't know.