Geordi's VISOR was because he was born without an optic nerve.
Bits of his brain were missing.
His parents had the same mum and dad.
They should have been able to clone new eyeballs for Tuvok and be done with it.
Here's the real drag.
Tuvok would rather be blind Tuvok and than a sighted Tuvix, unless he did ask Neelix to help a buddy out, and Neelix said "No".
Or maybe they went through with it, but Tuvix was still Blind.
"No, no, no, I'm back! I'm back! I don't mind being blind! You can't kill me again becau... I'm cool! I'm cool! Not like this, not like this AGAIN!"