To install Arch Linux to Sony Vaio VPCS-A3Z9R you need to make fake RAID work first:
# modprobe dm_mod # ls -l /dev/md
You will find all mdXXX device created. This prevents fakeraid from working properly, so we will fix it:
# mdadm -S /dev/mdXXX
Replace XXX by the values from ls command and start dmraid:
# dmraid -ay
All should work now and you can proceed with usual Arch Linux installation routine.
If you have any issues with screen not enabling after it gone blank try disabling display auto switch off in power management settings of your favorite desktop environment.
It is recommended to use opensource drivers from xf86-video-ati package. Somehow, vgaswitcheroo is not working, but switching graphics with PRIME works well.