Are you talking about the square plastic frame that fits up in the vent to cover the cut ceiling edges and the bottom of the vent assembly? I have a couple of cracked ones also, but haven't seen fit to worry about them--yet.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired