Broken Wheel Cover Clips cause vibration?

Hi All: I have been squaking about poor performace (vibration) form my new tires for days now. This morning I discovered that my front passanger side wheel cover had 1/2 of the plastic clips broken, on one side. Since the center locking bolt held the wheel cover on, I didn't loose the wheel cover. I should have left the wheel cover off to see if that made a difference regarding vibration. I was in a hurry and didn't consider that this morning. Is there a way to replace these clips short of replaceing the wheel cover? Also, these wheel cover are really dirty. I tried cleaning them and they still look lousey. Are there any good replacements available. This is for a 98 Grand Marquis