After a quick scan of the posts, I was surprised to see that the iron topic did not come up. I think that the Paula Radcliffe red meat article probably alluded to that, but I admit I did not read it.
I am a non-meat eater myself, so don't take this the wrong way.
Animal protein sources (especially red meat and organ meat) contain a form of iron (heme iron) that is more bioavailable than what you'll find in vitamins, supplements, and iron-fortified foods (breads, pastas, cereals, granola bars, etc). Iron is absolutely important concerning running performance.
I do not think there's a difference in where you obtain your protein. Vegans may have a bit of difficulty obtaining enough protein to fuel a running lifestyle, but as a vegetarian, I have no problem at all obtaining 1.5 grams per kilo BW on a daily basis. I don't even use a lot of soy protein. I am a dairy and peanut butter lover, however! As far as essential amino acids are concerned, vegans/vegetarians should have no trouble assembling a diet with complementary proteins unless they eat only a very monotonous diet (the same single food, all day every day, for instance).
Getting adequate iron is by far the larger issue when comparing veg and meat. I'd advise any active, vegetarian, of the age when they should be menstruating to have their iron status checked.
------------------
...on your left![/URL" target="_blank">
Positive thoughts--negative splits!