Wow, where to begin...
We tinkered around with our food system and 2/3 of the population is over-weight and 1/3 is obese. We suffer from heart disease, diabetes and related problems in epidemic proportions.
You think it's GM food that's making everyone fat?? What's making everyone fat is part chemistry (plastics) and other environmental changes (I'm 61, I never saw a single man with moobs when I was a kid no matter how fat he was) but mostly increased caloric intake. When I was a kid a small soda was 8 oz, medium 12, and large 16. Now a small is 20 oz. There were no half pound hamburgers. Restaurants didn't give you double portions of everything like they do now.
Maybe the solution isn't genetically modifying rice but something simpler as finding the right vegetables to grow alongside the rice that supplies the missing vitamin.
You might want to read some earlier comments; it isn't feasible.
Plus, vitamin A in excess is toxic and causes liver damage. Maybe we fix childhood blindness but instead give teenage cirrhosis.
You don't get vitamin A from food, you get beta carotine which the body converts to vitamin A, and your body will only produce as much as needed. You can only get too much vitamin A by taking artificial supplements or eating food that has vitamin A artificially introduced (breakfast cereal, drinks, etc).
Just because we can genetically modify plants doesn't mean we should go around looking for problems to solve with it
You don't have to look hard to see what vitamin A deficiency is doing to people in the third world.