I'm a vegetarian (not vegan) and do the quick service plan. I am usually able to find something to eat at most places, but it does take some planning so I'm eating more than just vegie burgers.
At Animal Kingdom their food choices suck in my opinion and they have moousse for desert everywhere which has gelatin. So for dessert I could get the kids cookie or fruit choice. Plus Flame Tree only has a fruit platter for the entree choice, that's not a meal in my opinon and Yak & Yeti has nothing, so besides Pizzafari we ate back at POP where they have lots of veg options.
EPCOt is better, I got awesome food at Norway, France, and Mexico.
Magic Kingdom is good too, Main St. Bakery has a cheese/tomato sandwich. Golden Oak has a veggie flatbread that I thought was gross, but Pecos has veggie burgers. Columbia has the lighthosue which is good.
Studios isn't as bad as some people say, Starring rolls has veggie sushi and awesome desserts, backlot has a veggie sandwich.
I prefer quick serve so the quick serve plan is good for me. If you do the one with the table service included you can find something at most table services places and if they don't have something on the menu they can usually make something else for you.
I ate at Mara a year ago and got a falfala (sp) pita which was good.