Post a picture and we'll be able to tell. In all likelihood: no. They have different faces/bezels, despite both being 4.0" AMOLED. But without being able to see it, I can't say definitely whether or not a Vibrant user would be willing to buy it, even at a discount.
EDIT: Oh, didn't this used to say Captivate? Whatever, disregard the above. I think a Fascinate's cover would work, but it might be a few millimeters short at the edges/corners. I'm not sure though; they appear to have identical faces.