Hey! I'll try to give a pseudo answer as I am not a vegan/not vegetarian, but you're touching on a few points.
I definitely see challenges with taking enough protein into your system if you're not eating meat. There are always lots of options everywhere for meat, and protein heavy 'other'foods are sometimes hard to come by. In addition, I've tried a hemp protein powder once, and it tasted awful :)
However, one way or another, I don't believe that the kind of protein matters (hemp vs whey vs meat etc) as long as you take enough in. If you're falling considerably short here, gaining muscle mass will be very difficult. Genetics definitely play a role, but I think only at a certain point. As in, if you don't have the right genes you won't be able to look like Arnold, but you can still be buff and muscular (without steroids etc).
I'm a strong believer that good results can be achieved with the right exercise and the right diet (unless there are underlying physical issues).
Anyways, my 2 cents :)
Edit: I actually wrote a bit about my personal opinion if protein powder itself is necessary for gaining muscle mass. Have a read if you're interested: https://steemit.com/fitness/@the-big-ginger/body-building-do-you-need-protein-powder