Absolutely get a GAC! Go to Guest Relations, explain your needs just as you did here, and they'll let you know the best accommodations. I would think at best you'd need to avoid stairs, otherwise you might present a safety hazard to yourself and others (if they try to go around you on stairs). I will say, unfortunately, that I have heard GAC for vision difficulties can be widely varied in how well it is handled at each attraction. But that is no reason to NOT request one. Also make good use of any other means you have available to you - like a white cane which signifies to others that you have vision difficulties.
Enjoy your vacation!