When people ask vegans where they get they protein from the best answer is were do herbivores like gorillas and cows get their protein from? Plants!
We do not need to eat animals to get enough protein in our diets, most people are consume way too much protein which is really bad for peoples health especially when its from animal based sources.
I get my protein as a vegan from vegan plant based protein powder, beans, nuts, seeds, leafy greens etc and its easy to get more than enough protein when you eat sufficient calories as a vegan and i have never personally had any issues gaining muscle mass on a vegan diet even with eating just one meal a day.