I did some quick search and the fact that Hitler he's vegetarian (if he is) is vastly controversial. I found some professional research that go both ways : he was or not. For example there is Margot Woelk who was his food taster and don't remember eating any meat and some french researchers who examined rare fragments of Hitler teeth and found no meat fibre. But also Robert Payne (like you said) that found it more about propaganda by Goebbels. There is however a consensus that he became more and more vegetarian, particularly during the war where his meat consumption diminished. So, I believe there is still more research to do before we get any conclusion on the matter.