I know that vitamin a is good for vision, but could it reverse bad vision in very high doses?

No. Vitamin a in large doses can be toxic and lead to a condition called pseudotumor cerebri. It is reversible with decreasing dosage of vit a. Pseuotumor causes severe headaches. More is not always better, it is just more. The body is designed to handle vitamins, minerals, etc in proper dosage for function. High doses will not reverse bad vision.