The wire clip presses a plastic tab forward; this puts tension on all the plates of the outlet grids and gives them enough friction to hold their position. If the plastic tang breaks off, it's flop city and there's nothing to hold the clip (you can see that he tang is missing from one of the two pieces in my middle vent, in the second picture). The fix is so simple! Just find a piece of plastic or wood about 1/16" thick (1 1/2mm). I used a piece from the end of a small spool of Radio Shack hookup wire. Cut it at an angle such that it will progressively push the plastic piece forward as you push the "fix" piece in. When you find the right tension, cut a notch slightly above the correct tension place; adjust the depth of the notch to apply the right tension when it's in place. After adjusting, apply 5 minute epoxy to the fix. After the epoxy sets, trim most of the excess from the fix piece.
Check one thing before you glue: Make sure the part that you insert doesn't interfere with the closing of the outlet flap. You might need to trim some material from the bottom of the fix piece to provide sufficient clearance.