You'll find that the food options are basically fast/snack food. Hot dogs, hamburgers, chicken strips, french fries, etc... We have annual passes to KSC and visit 8 times a year or so. There are not any vegan specific items at all however you can get a PBJ uncrustable or a small pre-made salad at ths Saturn V center.
Basically you'll have little choices other than above plus snack items like popcorn/ice cream and candy bars. Your best bet would to bring any food you would need in a back pack with you to insure you had food that you would like.
It's a fantastic area but really lacks in the food area.
Andy