Does your old openings have the *slide type hinging ? Or does it have the small multiple small hinge type?. They do make a slide on type hinge replacement cover which you dont need to replace the WHOLE metal plate.
Is there anyway you can post a picture?. Its sounding like you had a one piece vent unit,(Vent lid had intigrated hinges). All Vent covers have a *rain splash lip so when it rains, the water doesnt flow into roof. I can send you a link for Ebay which shows a slide on hinge type cover with a description and schematic.