M.L.
Hi L.!
I agree that it's up to you, but I also think there may not be one right answer!
My second child (a boy) is also visually impaired/legally blind. We knew we wanted a third child, but hadn't talked about the timing too much. Probably just going with three years apart b/c the first two were. And then surprise! I got pregnant when he was 12 months old. :P I was a little concerned too, but couldn't dwell on that since the baby was coming no matter what. And it all worked out just fine.
My boys are 21 months apart (almost 3.5 and 5 y/o) and I wouldn't have it any other way. They are such great buddies (yes they fight), but they are a barrel of laughs. The 3 y/o can help the 5 y/o with some things that might be tricky, and I can see how wonderful it will be for them to continue to grow up together so closely.
There has never been a moment where I felt that the baby compromised my middle one's needs in terms of his special situation. You will find a way to do it all!
Good luck, and go for it if you are otherwise ready now! :)