His eyes were "so" bad that he couldn’t read the number plate of the car in front. (Because "so... that" is a pattern.) The difference is when you use "very"; It is used to strengthen the meaning of an adjective or an adverb and can be followed by a noun. For example : Today weather is so cold. Today is a very cold day. He talked very slowly. When you use "so"; It emphasizes an adjective or an adverb to a higher degree than "very" and it can not be followed by a noun. For example : He talked so slowly. His eyes were so deep. He talked so beautiful that everyone got mesmerized.

So connotes more positive asertion than very, unlike too, which has assertion in negative sense