I've been following an organization that espouses a vegan lifestyle, but which does so non-confrontationally. The thing I have come to understand from this organization is that many vegetarians and vegans believe that ALL animals can think and feel and respond to human interaction. I have seen examples where chickens, turkeys, and pigs, among other animals, respond to humans much the same way a cat or a dog does. If you believe that animals think and feel and have emotions, then vegetarianism/veganism make sense. The basic question goes something like this: "Why do we love and protect cats and dogs, but eat chickens and cows and pigs? What entitles certain categories of animals to better treatment than others?"

I am not sure about this, but I think to some extent it is easier to feel sympathy/empathy for animals than for people. Animals cannot speak for themselves and the treatment many animals receive (in the factory farms that provide meat, eggs, and dairy to our society) is horrendous. So many people feel that they have an obligation to stand up for animals who cannot stand up for themselves. Sometimes it is harder to feel sympathy for people who may be seen as having done things to put themselves into precarious positions in life (doing drugs, drinking, dropping out of school). There may be some hypocrisy within the vegan community, but no more than in any other group, I would wager.

Can't address the first part of your question completely, but I think that they are so sensitive to suffering that it really truly distresses them to see those things.

Now the plant part is easy--that's what I studied in college. Even if you don't believe in killing plants, fewer would be killed to keep you alive than if they were first "processed" through an animal's body. Cows eat many many pounds of food every day. The soy or corn or whatever could sustain a lot more people if they ate it directly.