When they are this old, a number of things could be going on. It could be arthritis, it could be weakness in the hind end, it could be an inner ear problem, etc. Is he on pain medication at all? When my bunny was toward the end of his life, we found a daily dose of metacam really helped. He was old, so it was about making him comfortable at that point. If he has weakness in his hind end, it could be arthritis or a disc in the spine. If the circling has more to do with dizziness, then it could be an inner ear problem like an infection or EC.
Unfortunately, their immune systems start to not be so go at this stage. So opportunistic bacteria or EC can start causing problems. If it were me, I would start with pain meds first. Then move on to treating a potential infection. An MRI isn’t going to help at this point.
As for vitamin D, he should have been getting that from his pellets. A little bit of “sun” certainly can be good for them, but it isn’t so critical anymore with supplemented foods, etc. All of my bunnies have enjoyed natural light from the windows, but that’s it.