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.
loads of soy protein
Or watercress, spinach, sprouts, seeds, nuts, brown rice, protein powders made with the above, and many other foods.
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.
@rubashov: OP was talking about a vegetarian diet, not a vegan one. Whey protein should be fine. Whey is a by-product of cheese.
Whey's not vegan.
When I was in gain/build mode I was using brown rice and pea protein.