I read your question carefully and I understand your concern. I don t think that MRI shows BOTH diagnosis, I think it must be that the lesions can be either one or the other. To determine which one you must be examined and MRI reviewed by a specialist like neurologist and further tests might be needed such as lumbar puncture. Both are conditions which are worrying, if vasculitis prognosis depends on the type, if multiple sclerosis (most common demielinating disease), it is a chronic disease, not life threatening but can cause disability, for which you will need treatment for many years. I hope to have been of help.