Force breaking/fetch, trained retrieve, or whatever one wishes to call it has shown the most promise in an endeavor that has a low success rate. I have used it with some degree of success on a few dogs though fragile birds like woodcock and doves were still challenging on occasion.
Bumpers with nails or spikes, hairbrushes, frozen birds, and the like are pretty low on the scale. The suggestion of vinegar is a new one to me, I can't see it being any more successful than the above but can see a few potential problems.
Best of luck getting this straightened out.
