I actually had the opposite problem a few days ago (auto-completion and doc-popups not working, but error highlighting was). Everything is working now though, and I didn't try to do any troubleshooting at the time, so I have no idea what fixed it (possibly a system reboot, but I can't say for sure).
For reference, my VS Code is the latest version (1.22.2), and I'm using Nim 0.18.0.
