I agree with what has been written above. You can mention what your position is and why you adopted it, without being unduly pushy or preachy. There is no need to enter into arguments. If you discuss veganism with an acquaintance who just loves to argue (such people exist!
), then if you don't wish to argue you can just say, "You are entitled to your opinion, so am I. I'm not entering into an argument with you."
Regarding the issue of plants having feelings, I assume that you are referring to the work of Bose, Backster and others. There is a good summary here:
https://en.wikipedia.org/wiki/Plant_...8paranormal%29
The lack of proper controls in the experiments means that the tricky issue of whether plants can sense pain remains possible but very unlikely. I do not doubt that they do possess some form of awareness, perhaps full consciousness, but the ability to feel physical pain requires not just a nervous system (which they do have, albeit very different from humans/animals) but also a brain to sense pain signals. The question of whether plants have brains is discussed here:
http://www.naturalhistorymag.com/fea...ts-have-brains
The lack of a brain in plants to sense pain is really the basis for a vegan diet. Perhaps the Universe is planned in such a way, so that plants are aware but do not feel physical pain.