I have added tumeric to my gutload, also.
I don't think chameleons "choose" prey. They eat what moved and they can catch. They didn't think about it being a vertebrate or not and they don't think about the fact that they will get specific nutrients from one item or another, it happens by chance and my eating a variety of foods, that eat a variety of foods themselves.
Smaller chameleon species would easily find small frogs, salamanders, tiny ground geckos, and any number of non insect items. Preformed A is not limited to vertebrate prey. It's also found in the eyes of other Arthropoda.
I don't believe that insects have the storage mechanisms in their livers to store D3 and A, which is one reason they generally are constant feeders, because they need to eat a lot of food to maintain the proper levels of nutrients in their body.
I think chameleons can be kept without feeding vertebrate prey, just fine, but I also don't think that feeding appropriate vertebrate items would be anything but helpful, as the organs of small lizards will have natural preformed A and dietary calcium, D3, iron, and other nutrients. If I had the time and resources, I'd breed anoles and geckos and offer them as feeders on occasion, for all of my chameleons. I see value in it, as part of an overlooked, even if small part of a natural diet. Chameleons store A and even an occasional lizard snack could supply them with what they need, but would not get from an exclusively insect diet, in nature. 5% of a total diet could still have very important roles and we are fooling ourselves to believe a chameleon would turn down a prey item that they can catch and consume. It's not in a predator's nature to pass up a meal and even if the only reason they are consuming non-insect prey out of lack of insect choices, those prey items are still contributing nutrients they wouldn't get from onlyneating insects.
I don't think adding supplements to your gutload is harmful, but I think that is not a replacement for dusting the feeders, as the product is intended to be used, for guaranteed potency and delivery of the target vitamins.
I've never had eye problems, once preformed A dusting was implemented. I also believe humidity, misting duration, and other factors play key roles in eye health.