The part of a plant that has the most protein for you to eat and build muscle with is the seed or bean. There is lots of protein in the nuts, beans, and seeds, because the plant puts it there so that the baby plant can have plenty of protein as it starts growing. Soybeans have the most protein of any bean, except maybe hemp seeds, and tofu is made from soybeans and it is virtually all protein. Basically, vegans always get enough protein as long as you get enough calories and aren't starving. You needs vitamins for many important functions in all the different parts of your body.
Most vitamins are plentiful in plants, or are only really found in plants.
For instance, vitamin C is most abundant in citrus fruit, and vitamin A is found best as beta-carotene in plants that are orange, like carrots, sweet potatoes, squash, etc. So eat fruit and orange veggies for these. Zinc, as well, is found mostly in plants- in places like grains, nuts, and legumes. If you eat meat, you get it from eating the animals' blood. In plants, iron is used in the reactions of photosynthesis, so iron is found in the green parts of plants, where photosynthesis occurs.