Emma, I'm not so sure that the minute amounts of vit A that may be in stuff other than food would be the issue. I suspect it is the taking of daily vitamins and supplements where the most vit A would be restricted, second the food intake. Other than that, I think it may not matter. And unless you have a very strong issue with all forms of vit A, as with all the other possibilities, they just don't know for sure what triggers iih.
See how you feel by just reducing only food stuffs and supplements and see how that goes.