In "Cathedral," what is the significance of the blind man's lack of vision?
This is a great question. Of course, there is a massive irony in this story. Robert, the blind man that your question refers to, although he does lack vision, ironically shows that he "sees" far more than any other character. Consider his empathy and understanding that he displays towards both the narrator and the narrator's wife. Although the narrator is extremely rude to him, he does not respond in kind, but shows immense sensitivity and kindness towards both the narrator and his wife.
However, at the end of the story in particular, when he and the narrator draw a cathedral together, Robert manages to help the narrator himself to widen his vision and to see things differently, drawing a cathedral so Robert can "see" what one looks like. The final irony of the story is that the narrator himself closes his eyes too, so that he can "see" better too. Robert has helped the narrator to get in touch with his imagination and creativity by drawing the cathedral, in the same way that Robert's relationship with the narrator's wife inspired her to write poetry. The narrator feels "connected" to Robert, and the impact of this is profoundly liberating:
My eyes were still closed. I was in my house. I knew that. But I didn't feel like I was inside anything.
"It's really something," I said.