There are many individual plant protein sources that are not complete proteins, and some of the ones that provide complete proteins provide a low ratio of protein to other macronutrients and may not, for that reason, be suitable alone as a primary protein source. But there are plants that individually supply complete protein without this problem, and more importantly there are well-known and combinations of plant protein sources that do this -- they are often the staple foods of various cultures, since in pre-modern times very most agriculture societies didn't produce meat in sufficient quantities for it to be the main protein source, so if adequate plant sources didn't exist, neither would civilization.

(That being said, the heavy focus on soy in the market means there aren't as many convenient options meat-avoiding, soy-allergic/intolerant people as for either meat eaters or soy-tolerant folks.)

http://en.wikipedia.org/wiki/Complete_protein

Many plants have "complete" amino acid profiles but one or more amino acids are too low to be completely adequate for humans. In this case, another protein source with a supplemental amino acid profile is required in order to balance out the deficiency in the first.

You don't need to eat meat to get your protein.

So protein quality and quantity can't likely be insured by eating any old vegetables. Its not as simple as 'don't eat meat'.