Hi Scott,
I've had the Singh Ray Vario N Duo for a couple weeks and have been too busy to try it out. I just put it on when I saw your thread! The filter feels as thick as a brick...measuring about 3/4" thick!
To a large degree, vignetting will depend on the MFDB that you are using because of the crop and size of the sensor. (It is for this reason that I believe the P30/P30+ remains in that "sweet" spot as a great MFDB, using great lenses, not worrying about softness in the corners, and just enough high resolution for everything...)
I tried the Vari N Duo on a Mamiya 35mm and Phase 645 with P45+.
I'm not using the viewfinder mask. I could detect just a hint of a vignette in the corners, but that could just be my finger on the edge of the lens....
Just a quick down and dirty....but it may give you an idea. With the P30, I doubt you'd see any vignetting.... I'd be hesitant with the P65+
The Singh Ray Vari N Duo is a nice filter, but I won't be getting rid of my array of ND filters.....