Question: the command to launch OverdriveNTool -p0Vega0 -p1Vega1... is that pulling up separately saved profiles for each Vega card and applying them, or something else?
That's exactly what it does. Every card can undervolt to varying degrees, so I have different profiles with different voltages for each card. From the OverdriveNTool documentation:
Command Line:
-p[gpu_id]"Name"
apply profile "Name" to GPU with id=[gpu_id], silently