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 .