Many years ago, it was once thought that vegetarians had to combine certain foods in order to make a "complete protein." This is just a dietary myth that has been disproved. It originally surfaced in 1971 in the book Diet for a Small Planet, written by Frances Moore Lappe. By the 10th edition in 1981, Lappe renounced the myth as follows: "In 1971 I stressed protein complementarity because I assumed that the only way to get enough protein ... was to create a protein as usable by the body as animal protein. In combating the myth that meat is the only way to get high-quality protein, I reinforced another myth. I gave the impression that in order to get enough protein without meat, considerable care was needed in choosing foods. Actually, it is much easier than I thought."
Frances Moore Lappe was trying to encourage people to adopt a vegetarian diet and was not being malicious in spreading the original myth. She eventually realized that when we eat a well-planned, whole-food, nutrient-rich, plant-based diet, we don't need to worry about combining foods in order to receive all of the necessary amino acids.
There is a way to be protein deficient on a whole-foods plant-based diet ...by not consuming enough calories to sustain life. As long as you eat enough calories, protein deficiency will not be a problem.
For more information, the following are some excellent resources:
(1) The Myth of Complementary Protein by Jeff Novick
(2) Five Protein Myths by PCRM