Sometimes they are cloudy because of something other than Marek's. For example, cataracts. Sometimes they get a peck in the eye, and that gets infected and the cornea goes cloudy. I have had birds live a long time while blind, many years. Marek's is all around, so all your birds are exposed already. It might be too late to vaccinate, and they might already be resistant. Marek's in the eye gives a very irregular pupil, not round like usual, and not responsive to light at all. It doesn't get smaller when you shine a bright light at the eye.