This is correct by you don't want to disable the iGPU on rigs that use it (like mine).
You can instead call
devcon.exe disable "PCI\VEN_1002&DEV_687F"
timeout /t 2
devcon.exe enable "PCI\VEN_1002&DEV_687F"
This is enable/disable ALL devices that match the prefix.
Just my 2p

Even better! I disable the iGPU on all of my systems, so I wasn't really thinking about that. Can anyone confirm whether the vendor defined identifier (DEV_687F) is the same for all the Vega variants (V56, V64, V64L)?
These are Vega 56 from Sapphire.
You can push the wirldcard a little more, by using just PCI\VEN_1002. This will disable all AMD adapters.
I'm using devcon.exe from Windows 10 1607 (not sure if older versions have this wildcard functionality).