Beside total proteins and the need for protein combining (see this question), here I ask a more specific question regarding single amino-acids: what are the most likely essential amino acids to be deficient in a vegan diet? Where to find them?
A vegan diet is based only on vegetal foods. If we analyze the amino acid content of different food protein sources (animal and plant proteins), lysine is consistently at a much lower concentration in all major plant-food protein groups than in animal foods (1, 2). Lysine is one of the essential amino acids, thus lysine is most likely to be the first limiting amino acid in diets that are based predominantly on cereal grains (2).
For this reason the American Dietetic Association, in its 2009 position paper on vegetarian diets, recommends an increase of consumption of beans and soy products in order to satisfy the essential amino acids requirements in vegan diets (3).