Originally Posted by Nuke83
^^^^^This. Even the glues and epoxies specifically designed for plastics rarely hold.
OK, an addendum to my initial thought on this. I've used Shoe Goo for years to build up the toes on my sons cleats for "pitchers toe". Works great for that and also adheres very well to vinyl and rubber.
I installed VDP end caps this weekend to my front bumper and decided to run a thick bead of Shoe Goo between the bumper and caps, near the same line that the clips fit. The stuff never hardens, so it remains flexible, and should give it some extra support. I wouldn't trust it exclusively to attach the end caps to the bumper, but think it will be great in addition to the clips to providing a solid mating between the two parts.