Soy protein is an option (but not one for Alpha). Aaron Marino of alpha m. issues a disclaimer that the content of this video is his opinion and based on his research.
Alpha receives emails from vegan and vegetarians about soy protein. Soy protein is a complete protein but the 'problem' also has phytoestrogens. If you increase estrogen levels, you can interfere with testosterone. For this reason, Alpha doesn't supplement with soy. He does eats soy in small doses but supplements with non-soy products.