I spent about three hours at the weekend trying to make an ND filter for my Vision+ but in the end it wasn't so successful. The camera barrel appears to be 26mm so I took some 25mm conduit, cut it to around 0.75mm, reamed it out (sadly the wife won't let me get a lathe yet!) and then cut out a 25mm disk from an old perspex Cokin-type filter (ND4). Everything fit over the lens and the gimbal worked fine, once powered, but the image was quite washed out and the colour looked better without it on! Plus the fact that my smeary fingerprints were all over it didn't help.
Originally I had intended to use some ND gel sheets that I use for lighting control, that I could try to stick around the edge of the lens but I'm not sure how effective that would be. Does anyone have the camera specs for the Vision+ camera? I know that it uses a version of the H264 codec but reading other threads it appears that the shutter speed increases to compensate for bright light. This is not ideal for video cameras as you want to keep the shutter speed at twice the frame rate (or 180 degrees shutter angle) but I cannot find any specs anywhere that give this - any ideas?