I have also heard the same thing, that being vegan/vegetarian slows down muscles growth. I think this is primarily because of not enough protein. (Most Americans eat too much protein for their lifestyle, but I think you need it if you want to body build.) Anyways, it looks like people already posted some good resources, so you can find out there.
I just wanted to share a story (so gather round the magic carpet! lol)
I am a vegetarian that has been training for aerial silks. It took about 6 weeks to be able to do a pullup, which I think is longer than it would have taken a meat-eater, but by working on it about 10x a day, I got there. Here is me doing a pullup:
YouTube - Yay March Goal Yay!
So I think it is possible, it probably just takes more time, and maybe an extra protein serving. I drink Bolthouse Farms Perfectly Protein Chai Tea.