Good answers. It’s certainly possible. With a vigorous resistance training regimen (no more than 12-15RM) combined with eating a diet that provides complete proteins (grains + legumes, e.g., a peanut butter sandwich) it is possible to stimulate skeletal muscle hypertrophy. There are a number of vegetarian (consider including dairy and eggs) athletes who’ll attest to this fact.
Today there are more than enough vegan sources of protein that will result in muscle mass gains when applied properly in a sound resistance training program. Timing of meals and resistance training for strength/hypertropy are the most important things to consider. Of course, genetics is also a factor. But since you can’t change that, you need to apply the science behind the exercise.
Oh, and I have been eating a vegan diet for over 30 years. (Just never really liked milk and meat) I work with college and high school athletes who have trouble matching my energy and strength. And I am not a large person. It is not in my genetics. If you want more advice, contact me through my profile or website. www.hawaiifitnessacademy.com .