We never go wrong with the Bellagio Buffet. As vegetarians my girlfriend and I have previously greatly benefited from the chef counseling us on good gluten free and vegetarian options on the line.
Generally there are more than enough veggie options to choose from. Our last server "Evie" (sp) was amazing. She made our dining experience extraordinary. My only complaint is with the line attendant. We had an M-life card and wanted to pay the up charge to bypass the line however on two occasions they had no idea how to facilitate things.