There are probably some people who think that vegetarianism or green consumerism are necessary if insufficient conditions to save the world. I am close to one of those myself! (in the sense of the principles motivating vegetarianism, not necessary complete abstinence itself). And you could probably add me to the group who believe that liberalism is a preferable strategy to help the poor i.e., preferable to rapacious capitalism. I will even admit to being unable to significantly refute market socialism type arguments.
However, I think you will find very few vegetarians who think that their choice of food is the best method to save the planet. I don't have data here, but the sense I get is that most of them are motivated by [personal] ethical considerations (vegetarianism is not the means to an end), perhaps even aesthetic ones in some cases ;-).
--ravi