Vega needs the AMD display code, which unfortunately is not yet included upstream. (AMD is working hard on getting it upstream- the reason for upstream denial is HAL's in the code which take some time to replace)
For the time being, you will need to use the amd-staging kernel which includes the AMD display code.
To install the amd-staging-kernel, you have to simply ad
[mesa-git]
SigLevel = PackageOptional
Server = https://pkgbuild.com/~lcarlier/$repo/$arch
to your /etc/pacman.conf
and do a sudo pacman -Syyu
After that you need to do sudo pacman -S linux-amd-staging linux-amd-staging-headers
After that, for failsafe purposes, do a sudo update-grub
and reboot, in grub, chose the amd staging kernel
mesa and amdgpu already support Vega