Yeah you're right about the gatas and the veggie lasagne. It's never been directly stated on the tv ipakita though but they do imply that she's vegetarian or vegan sometimes. She's also worn some vegan clothes and Ezra mentioned something before about going to a vegan restaurant with her.
EDIT: In the most kamakailan episode, Aria tells Jake that she doesn't eat meat so it's confirmed that she is vegetarian.
Although it does say she is in the book, you can't really go sa pamamagitan ng that on the ipakita as the ipakita is often vastly different than the books. I mean, in the books, Emily is a freckled red-head whose hair has been turned kind of green and fried from all the chlorine in the pool. However, it is alluded to in the series several times; and she also tells Jake she doesn't eat meat. In the pilot episode though, she orders a cheeseburger at the Hollis bar.