How to remove an external dryer vent cover when it's covered by stucco?
The vanes in a vent like that are usually just plastic with small pegs that project out from each of the top left and right corners. These pegs fit into holes in the vertical side rail of the vent frame so that the vanes can hinge out when there is positive air pressure coming out the pipe. They then swing back into the closed position under the force of gravity. 

These vanes are removable by simply gently flexing the vane in the center and allowing the peg on one end to come out of its swing hinge hole. 

There is a very good possibility that you can find a replacement vent where the vanes are the same size. You could snap the vanes out of the new unit and insert them into the existing frame. Making this replacement would eliminate a lot of work trying to rip out the old vent and installing a new one.