I tested it myself, since I bought the camera and lens. The lens has to be very close to the eyepiece to lose vignetting. I used a file to remove the inner edge of the DCA adapter, so that it can be slid further over the eyepiece. Now it's fine. This modification could be useful for other lenses too.
I also tested something else. Compared to my Nikon 1 V1 + 18,5mm, I lose 2 stops of light (both wide open at same ISO). That was a bit disappointing, and I guess it's a result of the longer focal length (18,5 vs 35mm, both 50mm equivalent with their own sensor). With better ISO handling the Fuji wins 2 to 3 stops back.