Eye issues are fairly common cham troubles and can be caused by any of several things:
1) Infection, which can be due to injury, environmental issues, nutritional deficiencies or parasite problems
2) Malnutrition which can be caused by failing to gutload feeders or failure to feed a varied diet, improper supplementation, parasites or simply a higher need for a particular nutrient in an individual chameleon
3) Irritation--sometimes a cham's eye is irritated by something in their environment, such as too little humidity, sap from a plant or a bit of something getting into the eye (shed skin, soil particle, etc.)
The first 2 problems are best treated by a vet.
It is important is to carefully review your care, whenever a health issue arises, as even infections are often caused by environmental issues.
For the third problem, gentle, repeated flushing of a cham's eye with a sterile saline solution designed for use in eyes will typically clear things up, but if there isn't substantial improvement within a day or two, it's time to see a vet.
FWIW, I've used this one
http://www.bausch.com/en/Our-Produc...Lens-Care-Products/Sensitive-Eyes-Plus-Saline
It goes without saying that if there is something in your cham's environment that caused the irritation, it should be remedied.
Vitamin A treatment at home can easily do more harm than good because too much vitamin A damages the liver/kidneys, as well as doing other undesirable things.
The safest way for a corrective dose of vitamin A to be administered is by an experienced reptile vet--who will inject a measured dose of vitamin A.
The danger of simply trying this or that is possibly fixing the problem only accidentally and temporarily--or in the worst instance, delaying necessary vet treatment until permanent damage is done.
Here are 3 more posts about eye troubles and other health issues:
http://seavs.com/lizards/chameleons.html
https://www.chameleonforums.com/blogs/ataraxia/404-common-eye-problems-solutions-rev1-1.html
https://www.chameleonforums.com/eye-problem-27421/index2.html