"Black Glass" vignetting issue with the RX1
Since I posed my question a while back I've done some more research as well as testing. At the 35mm FL underexposing with a 10 stop ND causes vignetting (its a mathematical thing, I wouldn't understand). I was underexposed and did not know it (the in-camera exposure compensation data is of little value when using this filter... and did not review the histogram after the shot). I was maxed out at 30 seconds shutter speed and had no cable release... but I did not pick up on the vignetting until PP anyway.
Now I know I am not stuck with vignetting with this ND filter for all my shots (if anyone cares).