HI S.,
I noticed that a lot of the people responding were referring to a vegan diet as opposed to a vegetarian diet. There is a huge difference between the two. I fully support your daughter's choice be become a vegetarian, however, I would be wary about a vegan diet for a young girl simply because a vegan diet is one containing no animal products at all... including meat, poultry, fish, milk, eggs and cheese. I think it's a hard diet to maintain for anyone... and there has been some concern about eating too much soy.
Having said all of that, I have been (mostly) a vegetarian for many years and have raised my two children 3 and 5 on a vegetarian diet. They are perfectly healthy.
If your daughter likes a variety of foods and spices that is a good start. There are lots of cookbooks and resources online to help with ideas of how to prepare tasty veggie meals. Nuts are a great source of protein and easy and filling to pack in a lunch. There are some great bean salad recipes that would pack easily. ( I have a great one if you're interested!)
I would be cautious of all of the processed simulated meat foods. I think they are ok in moderation, but some are loaded with "stuff" and SALT!!
Hope this helps. Good for you for listening to your daughter and supporting her. At 8 years old she has her own opinions about what she likes and who she is and whether she chooses to continue with this diet forever or it's just an influenced phase as some people suggested, she'll appreciated that you let her make her own choices and kept her healthy doing it. Also, what a great opportunity for you two to learn together about a healthy diet... especially in our society of obesity, diabetes etc...
J.