Vegetarians will most likely feel very left out at most SCA feasts. If you are vegetarian, the feastocrat will do his/her best to ensure there are some yummy veggie dishes, but most times you will most likely find there are mountains of meat with a small pocket of green vegetables tucked somewhere to the side of the serving plate, floating in a sea of delicious meaty, fatty juices.
There are many period vegetarian recipes, mostly from religious fasting days, or Eastern cuisine. Most feastocrats are understanding about dietary requirements - just give them plenty of notice to let them adjust the menu. Alternatively some feastocrats will cause vegetables to be separately served, giving their populace a chance to taste vegies free from dead pig juice.