When I had my vitrectomy/gas bubble I was told to come in immediatly if I noticed any more dark patches on the periphery of my vision. I noticed a "blind" spot in my top peripheral vision within a couple of days and rushed back in. Turns out it was the boundary between the liquid and the gas in my eye. As the gas bubble shrank (so the boundary rose in my eye), I perceived the dark patch sinking. At the end the tiny gas bubble looked black and appeared to jiggle around at the bottom of my eye.
Recovery from a vitrectomy, particularly with a gas bubble, is weird. You will perceive all kinds of effects. I panicked more than once, but the docs kept telling me my retina was well attached, and they were right.
Think about it this way - you've already had cataract surgery in that eye, so that's one less thing to worry about!
Good luck.
Thank you so much, Doctor Hagan. That already makes me feel better. I had tried the search function but didn't find much of people seeing things like a half circle or ellipse after vitrectomy. Maybe one or two mentioned it. But I will try the archive search. Thank you!
One more note, I have had cataract surgery in that eye three years earlier.