There are many vegetarian Buddhists here in Taiwan and their food is sadly well-known for being overly greasy (as to add flavor). It doesn't help that the local Buddhists avoid onion and garlic, two staples in non-vegetarian local food. It's considered normal to eat supplements, but that's of course a recent phenomenon.
I'm not saying it's all bad (I'm mostly vegetarian), but the studies are all over the place as with everything in nutrition :)
There are so many varieties of vegetarian diet the term is a little meaningless in terms of understanding nutrition.
I've been vegetarian for about 20 years, I eat a lot of fresh veg, not much fried stuff, not much processed stuff, try and make sure I mix things up and get some protein. Broadly speaking I think I've got a pretty good diet.
But I also know vegetarian's who are massively overweight, eat crap (and too much of it), drink too much and so on.
As a broad group I suspect vegetarian's eat better but I think that's more likely to be linked to the fact that this is a group of people who have thought about what they eat as much as anything inherent in the lack of meat.
And that would be interesting (and I think they do exist - I'm curious about the poster above saying that they show it's not healthy because my understanding is precisely the opposite), just I think that there are other factors you need to control for to really understand it.
Someone better warn the Buddhist monks, they've been at it a while and if only they'd known they might have lived a bit longer.