I don't know how it goes for you but in my case, if things go to shit, it's always when I answer why I'm vegan => because I don't want to hurt animals. From then, I can say whatever I want, it's too late, I'm forced into a moral superiority contest even though I don't give a flying fuck about where I stand on the morality ladder.
For me, it's like flipping a coin. In 50% of the time, my encounters play out like that - or they would play out like that if I was pushing for further discussion. The other 50% people seem to be genuinely interested and ask questions. However, they mostly want to have a conversation but are not in a mental state where they would want to "understand" what I am saying.
On the other hand, I have some training in discussing vegan topics, so I am slightly prepared to not let discussions derail that often.