While most vegetable protein sources do not provide a complete protein source I believe a couple of sources do (or at least one that I remember), namely soy beans (check it out on http://en.wikipedia.org/wiki/Soybean).
"Soybeans are generally considered to be a source of complete protein, without any need for Protein combining.[28]"
So, Bellotreeman is correct on this point.
Also, while other vegetable sources are generally lacking one or more amino acids they are not necessarily all lacking the same amino acids. So, by eating a variety of vegetable sources you can actually obtain the full spectrum of amino acids necessary to function.
I also seem to remember their being 8 essential amino acids and I think the body can synthesise the rest, so you just need to make sure you get the essential ones in your diet (you'll need to check on this though).
