Willingness to consider ideas that at first appear wrong. Without this, our knowledge would never contain the counterintuitive. If we limit ourselves to what's intuitive, then we severely limit our intelligence. The connection to vegetarianism is that our culture teaches us that animals are objects that we can use as we wish, so vegetarianism at first appears wrong to us because of our social upbringing.
A sense of duty toward the truth. Gandhi was like this: he was very driven toward the truth and very accepting of the outcome. As a result, he was always performing many "experiments" and continually learning. A sense of duty toward (and lack of fear of) the truth will expose you to many new experiences and allow you to incorporate new information, which can increase your intelligence. The link to vegetarianism is this: many meat-eaters I know are not meat-eaters because they sat down one day and made an intelligent, intentional decision to start (or continue) to eat meat, they either have not yet been exposed to different ways of thinking about animals or they have been exposed but they avoid the issue.
A desire for a consistent world view. There is an inconsistency in our culture in the way we treat some animals (pets) and other animals (factory-farmed livestock). Michael Vick's dog cruelty is punished but the same animal cruelty is condoned for the sake of food. There is a logical contradiction here which takes some intelligence to notice. This, I think, is one of the real reasons why there can be a link between intelligence and vegetarianism: there are less philosophical contradictions in a vegetarian's position. If you put a meat-eater and a vegetarian head-to-head in a philosophical debate, my money would go on the vegetarian.