Connect with vegetarian and vegan friends from all over the world.
You do not need to argue with them. Those who want to understand, will listen to you. Those who don't want to understand, will never listen to you with the intent to understand, they only listen with the intent to reply, to argue back to you. So please just ignore them.
Just keep promoting veganism, someday, those who want to listen, will understand, and will turn to be a vegan like you too.