Had the same problem but broke both Driver vents. I used a modified Curtain insert (adjust so it fits the vertical slot width). I worked the modified tool so I could slip it in, aligned and put slight presssure (pulling towards me) and snapped them back in. However on the Driver side right vent (middle vents), I noticed the tiny metal piece was broken so it could pop out again. It works functionally. Just gotta make sure it I don't pull too hard.
Can't believe BMW would cheap out on the vent sliders. Of all the cars I've ever owned, this has never happened.