Visit Home Depot, or somewheres, and buy a roll of 1/4" or 1/8" wire mesh. It's usually on the shelves outside in the Garden Section. Then, buy some steel wire and wire the mesh to the existing screen on the vent, and you're finished.
My best to ya and hope this helps.
Jay J -Moderator
PS: If the openings are too large, use 2 layers of mesh where 1 layer 'splits-the-difference' over the 1st layer of mesh. Tie each layer to each other, then tie the 2-layered mesh to the screen on the vent ...