I think that something that has been only glanced-over that would do a lot more to help with the problem that livestock poses than going to a vegan diet... is that which Asta linked to.

There are many species that are invasive and are doing untold damage to the environment. Let's eat them. I'm sure someone wrote a book about it, even. Eating invasive species would solve the problems they're causing, as well as reduce our dependency on livestock. I even heard that pigeons (which are a VERY invasive species) are also edible. It could replace chicken. I don't think a vegan diet is the solution. As far as I know, vegan diets require the use of supplements. Any diet that requires the use of supplements doesn't strike me as a good idea.