The issue with this joke is that it's 100% confirmation bias. You know that all the vegans tell you they're vegan because you don't know about the ones that didn't.
There are people that I discovered were vegans when discussing what to eat. And there are people that loudly announced they'd become vegans without any context.
The later group used to be larger, but I don't remember anybody doing that recently.
I think it's also survivorship bias. All the people who tell you they're vegan being vegan does not mean that all the people who are vegan will tell you they're vegan, just like all the people who won the lottery probably having bought lots of tickets does not mean that buying lots of tickets will win you the lottery.
Confirmation bias typically refers to accepting very little evidence or rejecting disproofs. Survivorship bias is the one where you forget to account for the non-survivors of a particular selection criteria.
I know this under "selection bias", so now we have three contenders.
Still, I'm not sure if it's a big factor. You likely know who is vegan in your circles, because veganism is unusual enough to be a good topic for gossip, you can easily see it when sharing a meal with someone, and it's also something you'd like to know when inviting people over, if you want to be a good host. So I think a more relevant estimate would be number of vegans you know vs. how many of them announce their veganism at the earliest opportunity.
It made more sense 20 years ago when it was more generally true. Veganism seems to have spread beyond the merely self-righteous now. It's a very old joke (possibly from the 90?s).
