Even IF, I would not use a vegan diet on whatever carnivore pet. The reason for this is, except from the fact that I think its is sick to even think about doing something like this, that i have a naturally grown, unprocessed and healthy product available (mouse or rat) and the alternative would be a highly processed piece of crap from the labs of the nutrional scientists (modern Frankensteins).
I have been vegetarian for around 8 years, but i always avoided all this "fake meat" and I would not want my snakes to eat that stuff.