Again, I'm speaking from a different model than yours, but I do recall that the vents were a REALLY snug fit! Also look inside and see if there is a screw holding it in place on the left or right side. Mine was anchored on the right side, and I didn't want to remove the airbag to get to it, so I bent the bezel away from the vents and watched the vent pieces fall to the floor!
I don't think that will be the case for yours, though...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car