Hitters need to see a 4 inch ball moving at 90-100 MPH. Dak needs to see a 6 ft 200 lb receiver moving at about 20 mph. He'd have to be legally blind before his eyes would be a problem.
Dak's problem is in his head. He just is not making quick decisions or the right decisions. You don't have to have 20-20 vision to throw the ball out of bounds when nothing is there. or to se the play develop and take the dump off right away instead of waiting until even the outlet pass is not an option.
Honestly, he is just a mess right now. The one quick pass he threw trying to hit Schultz on a short route he threw right into the hands of the niners LB, who luckily dropped. it. I don't think he gets better next year. I think all these negative plays eat away at him and he regresses further.