I haven't RTFT yet, but I had to respond when I read this:
I imagine it will fizzle out pretty quickly. Especially once she actually tastes most vegan food.
I suppose, being kind (and I always try to be kind to the less informed), you must have had bad experiences with vegan food. That's obviously your bad luck, since most vegan food is excellent.
Then I read your post again, SlottedSpoon, and came across this little gem, which at first I was prepared to let go:
Sorry but if my child of 12 wanted to become vegan I'd be making it clear that they'd have to do all their own cooking from now on. That should cure it pretty sharpish.
That is such a revealing statement which tells us far more about you than you realise! 
Firstly, it's completely prejudiced - and there's an evident lack of respect and empathy for your daughter and her choices.
Then you show your lack of understanding of vegan nutrition:
I simply do not believe that veganism is a sensible or healthy way to eat, especially not for a growing child about to go through puberty.
A read through this thread would go a long way to addressing this. 
But is that enough?
It seems to me there are three reasons for going vegan - each one of them sufficient in its own right:
Concern for your own health;
Concern for animal welfare; and,
Concern for the planet:
4 families raising vegan youngsters.
Vegan Kids
Novak Djokovic is a vegan - in fact he's opened his own vegan restaurant in Monaco.
Bill Clinton went vegan to avoid having another heart attack.
It's the future, folks! 