the sensor might not be aligned, you could be putting it on crooked, most fisheyes are a lil bit uneven. any of those really. if it bothers you just zoom it in a lil bit to get rid of it.
Zoom a little bit, thats your only option. Vignetting is usually in the overscan so when you watch it on TV like your supposed to most of the vig would be cut off of the picture (with SD)