Probably the only possible way to put on muscle mass as a vegan is with loads of soy protein. Which is pretty much toxic. Have fun with cancer and endocrine problems.
There's whey protein as well. That's what's in Muscle Milk and Pure Protein products. I'm not sure if whey protein is any better for your endocrine system, FWIW.
Whey's not vegan.
@rubashov: OP was talking about a vegetarian diet, not a vegan one. Whey protein should be fine. Whey is a by-product of cheese.
loads of soy protein
Or watercress, spinach, sprouts, seeds, nuts, brown rice, protein powders made with the above, and many other foods.