Well, truth be tooold, I kinda cheated. The collar is made of aluminum, BUT the dent that's in the chest piece, I used a dremmel without any attachments (cause I didn't have much to choose from except the ones for cutting). I just kinda went back and forth until I got a nice little dent, be careful not to overdo it though! Sure it makes the plastic a little bit "weaker," but it's not like you're going to be taking real hits on the stuff. Hope it helps. Oh, I've also heard using a heat gun works as well. Just brainstorming right now -- couldn't you super heat one of those smaller hammers with the kinda rounded ball heads (not the normal hammers w/ the flat heads) and then press it against the plastic? (shrugs) Good luck on it either way.
:cheers
Miggs
PS: I Velcro'd my collar to the vest, I'm not much for ripping armor off if I ever wanted to ugrade. (added a little double sided tape for extra security as well)