Fasting helps blood sugar levels and diabetics are known to have less than optimal vision so I think it makes sense that vision improves with fasting. The Reverend Al Sharpton lost all that weight a while ago and spoke about how his vision has improved because of it.
That’s perfectly normal, you’ll probably also notice your mind becomes sharper and your hearing improves too. It’s just part of evolution and a survival trait. When you’re fasting your body goes into “hunter gatherer” mode and your senses heighten to make you a more efficient hunter.