While definitely not a valid reason to dismiss veganism, in my experience, I can understand why that stereotype exists. Still, it’s just a stereotype, and omnivores need to remain open minded and acknowledge it as such.
"Vegans are preachy" isn't an argument. It's a dodge to avoid talking about the real issues. When people were protesting the Yulin Dog Meat Festival, nobody was saying "These anti-dog-meat-eaters are too preachy! Maybe if they were more polite, they would change more minds."
Yup. It’s not an argument. It’s an attempt to derail the conversation, and/or make the vegan angry so they can say “ah ha! I knew you were irrational!” and then walk away feeling superior.