Proposal: Veganism and Vegetarianism
Would a nutrition and diet site not be more useful? Questions relating specifically to veganism or vegetarianism seem too specific.
This site is targeted towards a specific audience by intent. The broader site you mention would be addressed by the two existing sites Health and to lessor extent by Seasoned Advice. There is not an existing site that covers the ethical/religious aspects of Veganism and Vegetarianism, which is an import part of this sites scope. See related Should Ethics be allowed?
In Summary, No. A broader nutrition and diet site is would not be more usefully for the intended audience. That need is already met and does not adequately meet the needs of this audience.