The omnipred is a corticosteroid used to reduce inflammation, these drops are delaying healing so that the antibiotic drops (vigamox) can heal any infection first. These drops are typically used for treating corneal ulcer. When you go for your follow-up visit with the eye doctor he will examine your cornea to be sure that it is healing properly.
There are different possible causes of the ulcer, and the doctor should have mentioned what might have caused it. Do you wear contact lenses?