All schools have some statement on their "technical standards" for admission. They include things like observational skills, communication skills, motor function, intellectual abilities, and behavioral/social attributes. Reasonable accommodations are made for disabled students, but a physical impairment that raises serious concerns about your ability to independently perform as a medical student/physician are acceptable reasons for denying admission.
Vision impairment is not usually something that keeps you from doing what you need to do, though. I know that there are quite a few blind doctors, although I'm not sure if they were blind during medical school or if it came later. You probably need to at least be able to read a chart/computer screen and adequately observe a patient at bedside.