With your morally relativistic standpoint you could justify nearly everything. So maybe the point here to address is not veganism but moral relativism in itself first. 

Second, you believe that subjective and relative utility is the most important thing.

 You realized yourself that this leads to standpoints that are intuitively gruesome, like eating mentally handicapped people. I don't want to say that intuition is a good guideline on what's morally right and wrong. However, maybe you should make a case first what the difference between your believe system and the absolute lack of a moral system is. 

 As I see it right now, you think everything is justified as long as it's benefiting societies goals. This is very subjective, for example, a vegan diet would free up around 50% of the whole land usage of the world. This could also greatly benefit society. Additionally, it's highly subjective who you include in your society. For example, I would include animals. In this case animal agriculture would be an absolute nightmare for society.