A full-frame image sensor is based on the size of 35mm film and therefore has a 42mm diagonal. This corresponds to a 21mm radius circle at the image plane.
Yes, there is vignetting. Just like any real optical system, an EdgeHD OTA doesn’t have 100% illumination across the image plane. In other words, the EdgeHD optics don’t fully illuminate the 21mm radius image circle.
With the reducer, the vignetting falls to a value of about 40-50% at 21mm. The exact value depends on the specific EdgeHD OTA (8, 9.25, 11 or 14 inches).
The problem of vignetting in astroimages is usually handled by the technique of flat fielding during post processing of astroimages.