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my Vari-ND Duo has never blocked as much as 5 stops of light, let alone 8 like the original Vari-ND. at higher densities, it has uneven polarization even if the density is even. i've done enough testing to know. shooting water without a polarizer compromises image quality too much because you don't get enough reflection control. i barely use mine, prefering to use 3 different B+W solid ND filters stacked with a CPL. together, they cost about the same as the Vari-ND Duo and get much better results. the vignetting issues are completely lens dependent. you have to find out from someone whether the Vari-ND Duo will vignette or not because they have used it on the same lens.
The Singh Ray Vari-N-Duo is the ticket. Sort of. It gives you up to 8 stops of ND (which you will need) plus polarizer and it is the polarizer is that takes it to the next level. Besides being expensive, here's the problem. It only comes as a 77mm and even in the thin mount will vignette badly up to about 35 mm on a 24-105. I've been told that the same is true on a Nikon 24-70.