The vegetable wellington at Steakhouse 71 was really good. So good I went back again the next trip just to have it. Also, as weird as it sounds, I've yet to be disappointed at Flying Fish. I went the first time to appease the rest of the family who all love fish, and liked it so much I went back alone on my next trip. Each time they have a great vegetarian option.
Casey's Corner in the Magic Kingdom has a fake sausage link that has some good toppings. And Satuli Canteen in Animal Kingdom is fantastic!
Honestly, I don't find that I am just stuck with veggie burgers. I go a few times a year and sometimes at the end of the trip I realize I haven't had a single Disney french fry (which I love, but normally only get when I eat a veggie burger).
If you are on Facebook check out Vegan Disney World. While I am not vegan, it is a great resource to let you know what is available.