'unless you've seen someone' is not a good standard to use here, because of course some people are like that, that's where the stereotype comes from - but the vegans who don't 'keep going on about it' are inherently not as visible: it's not often you'll know someone is a vegan and not talking about it, essentially never for strangers.
So if you really care about reasoning with people to challenge the stereotype, you need to know % vegans (according to some survey or whatever) in a population relevant to a person so that you can ask them to consider not if they've ever seen one vegan talk about it, but if they've seen a number X close to that % of the total people they've seen talk about it (and if not that they've encountered that-X% extra vegans without realising it).
So if you really care about reasoning with people to challenge the stereotype, you need to know % vegans (according to some survey or whatever) in a population relevant to a person so that you can ask them to consider not if they've ever seen one vegan talk about it, but if they've seen a number X close to that % of the total people they've seen talk about it (and if not that they've encountered that-X% extra vegans without realising it).