How did Richard Parker gain his vision again in Life of Pi?
Well spotted! After perusing the relevant chapters, I can say that you are right to express confusion about this, precisely because the text never tells us how Richard Parker does regain his vision. We are told that Richard Parker is blind in Chapter 90, when Pi looks into his eyes "like an eye doctor" and Richard Parker looks back vacantly. As Pi says, "Only a blind wild cat would fail to react to such a stare." This of course comes just before Pi himself goes blind and the encounter with another mystery castaway on the ocean, whom Richard Parker kills. The next reference we are given to Richard Parker and his sight comes when they reach the carnivorous island that serves as a shelter for them both for a while, and we are told that Richard Parker is seeing again perfectly fine:
Richard Parker was observing me from the lifeboat. He was looking at the island, too.
No reference is made as to how he regained his vision, but we can only assume that at some point during Pi's own episode of blindness, Richard Parker regained his sight.