The vegetarian food pyramid
There is a role for vegetarianism's compassionate food choices in all forms of Buddhism. Mahayana tradition already incorporates this. (Vajrayana and Theravada, for the most part, do not). But in order that Buddhism would be able to spread throughout the world as a "universal" set of practices and doctrines, there was no injunction that one must eat this way or not eat that way.
Everyone is free from a Buddhist perspective to choose one's own actions (karma) and meet with the results of that freewill. Compassion can guide one to doing what is best for oneself, and best for others, and best for both (i.e., the environment). The choice is not either/or but good for all three. Salagram is a fabulous resource for an understanding of vegetarianism from the perspective of many religious traditions.
Veganism: living with no animal products at all