Sally, sorry to sound harsh but unless she is undertaking this bizarre lifestyle for medical reasons, can you not just tell her to get on with it her own way?
If you are financially responsible for her then give her a percentage of what you spend on the family food and tell her to shop, cook and eat herself??
As a mother I am sure you have better things to do and a busy schedule without having to worry about cooking Tofu casseroles.
Remember vegans do not eat eggs, milk, all forms of gelatine etc etc etc.
As you can tell I am very intolerant of bloody veggies, let alone vegans!!!!
Meat is healthy, meat is good for you and meat tastes good.
However, if you think my advice is pointless please monitor your daughters health. I do not care whatever pro-vegans say, they look ill, undernourished and often very pale!!!