World Hunger: Approximately 9 million humans die of hunger and hunger-related diseases every year. We already produce enough food on earth to feed around 10 billion people, but the majority of grain crops is fed to livestock. Being vegan can effectively contribute to ending world hunger.
I don't think a population level shift to veganism would have the effect on world hunger that you are presenting here. The problem is availability. If the grain being fed to livestock is suddenly freed up, I don't know that it is just diverted to people experiencing hunger.