I don't really know what you mean by "vision", but the most and fastest proven way to improve one's handwriting is constant practice and a goal why you need to improve your writing..
I myself used to have the worst handwriting. My handwriting was small and they are written so close to each other that you one needs to take a really good look at it to read my writing. However everything changed when I went to college and have this "Drawing 101" subject in which we were asked to almost always use straight big letters(Kind of like the Times New Roman font). In Engineering, this is quite a must in order for everyone to easily read what you wrote.
Long story short, because of that 1 semester subject of mine, I learned (or was forced) to write in Big readable letters. My goal at that time was to pass the subject and the only way to pass that subject was to adhere to the rules given by the professor. And the practice was a good full semester.
So if you want to improve your handwriting, you must first ask yourself why do you want to improve it. Are you forced to or do you really want to improve? With vision alone it is hard, as "handwritings" are personal stuffs that no one other than you notices especially in this age of technological advancement.