Fisheye question

Shot these on my Pentax k1000 while trying out a fisheye and flash combo for the first time. The vignetting seems to be off center, very heavy on the bottom compared to the top, and somewhat inconsistent. I am wondering if there is a solution to center the vignetting for my shots?

I shot these vertical, so the flash should be coming from the left side of these photos. The lens sticks out further than the flash. Not sure if this has anything to do with it. Or if the lens isn’t catching enough light from the side closest to the ground, causing vignetting heavily on that side.

Any insight is heavily appreciated.