One thing that seems to make sense to me about the barrel-bolt is this. If the clip gets bent to the point of being loose, while it is still screwed on tightly to the knife, then you may have a problem with looseness. Understand that the part of the clip that rests into the recess of the FRN acts as a sort of lock washer. Its rectangular in shape, which prevents the clip from slipping side to side. If the clip gets bent so that the whole of that rectangular section is not resting perfectly within that recess, then you will not be able to make it secure because the shaped part (rectangular part of the clip that rest in the recessed FRN) is no longer the proper shape (as in flat) to maintain that security.
This is important because if the clip gets bent away from the handle while the barrel-bolt is tight, then chances are that it was bent at the point where the barrel-bolt screw is holding it in place. That means that the clip is now deformed in that area, and will not hold it securely (IOW, it will slop around).
This doesnt account for it not staying in your pocket. No matter how loose the clip is, you should be able to take it off, and bend it to the point that it stays secure. You should be able to get it so tight that you might actually think your trying to draw a Subcom F out of your pocket when you go to draw it.
The Vice-Grips is a good idea because the nose of them is flat and square. Clamp it right up against the part that rises out of the recess, and then bend the clip.