As a constructive comment, with no proof of fact behind it, but perhaps when vaccinating a feather follicle has been damaged causing it to twist / curl. I don't think it is to do with the one spot treatment because if it was, it would have happened when the feathers were pulled back to reveal the skin. I'm sure you've tried the simple fix of turning them back round. This would bring you back to the vaccinations and follicle damage. I know follicles are for hair, but don't know the correct term for feather growth spots. Only other alternative is some form of external parasite, but they have been treated for that. No facts, just guesswork which I'm sure you have already thought through. Hope someone is able to help in a more decisive manner.