Getting Linux Mint 18.3 to work with AMD RX Vega
This worked for me at least (February 2018). No guarantee it will work for anyone else though. It also depend on driver versions on repositorysInstall Linux Mint 18.3
System is now in software rendering mode.
Run in terminal
sudo add-apt-repository ppa:teejee2008/ppa
sudo apt-get update
sudo apt-get install ukuu
Run Ukuu and update kernel to 4.15 (older versions does not natively support RX Vega)
Go back to Terminal and run
sudo add-apt-repository ppa:ubuntu-x-swat/updates
sudo apt-get update
Restart
Run Update Manager, use default setting "review sensitive updates"
Choose to also install the updates on Level 3 and 4, including updates to Mesa.
Restart
The system should now work, I have had some issues with stability (system locking, had these before but more frequent now), and with NTP (lot of solicitating NTP servers, might be unrelated or bug in underlying Ubuntu 16).
Regarding the Ubuntu-X Repository - You may want to know what it is before you rely on it for your Xorg/Mesa updates
From what I have heard it is a slightly more official, inofficial Ubuntu PPA, with new but less bleeding edge drivers compareds to for example xorg-edgers. Link to the Ubuntu-X PPA