Is Vega High Sierra only?
Can I use Vega in Sierra somehow or does it have to be High Sierra? Because HS is really buggy on my laptop, all sorts of glitches and sleep issues.
The drivers for RX Vega cards are only present in 10.13 High Sierra. It's not possible to use Vega in 10.12.
That's unfortunate. I have a 2016 MacBook Pro 15" and trying to run in clamshell mode with a usb-c monitor has been a nightmare. Frozen system with grey internal screen, graphical artefacts, it would simply crash during sleep mode and show me an error next time. Closing and opening the lid again causes the whole monitor to go completely yellow. Actually Sierra is not perfect either but it's better with these issues.
Vega drivers are not ready yet. They will be High Sierra only.