There is nothing that will ruin a good impression quicker than modern eyeglasses. Of course, if you are nearsighted it won't do for you to go without your glasses. Remember, you are handling a loaded gun and need all your senses to do it safely. So, either you should look into contact lenses, or you can order a pair of period eyeglasses - just for reenacting. You can buy a pair of reproduction frames at many sutlers or you can hunt up and purchase antique frames if you prefer. You won't be able to take them to just any optician. Many period frames do not have adjustment screws to hold the lenses in place. Getting the lenses ground to just the right size and into the frames is an (almost) lost art. So send a current prescription along with your frames to an optician who has experience working with period eyewear. If you can't find frames, some opticians can provide them for you. Expect to spend just over $100 altogether.
By the way, if you have a Ferrotype or Ambrotype taken of yourself in your uniform, don't wear your glasses. 19th Century gentlemen usually removed their glasses for a portrait.
19th Century Reproduction Eyeglass Frames:
Jas. Townsend & Son, Inc.
133 North First Street
P.O. Box 415
Pierceton, IN 46562
Optician Able to Work with Period Frames:
Sun, Sports, and Vintage Eyewear
70 S. Oak Street
Ventura, CA 93001