It's very dubious to be putting our efforts advocating this as the "single biggest way", certainly it's terrible marketing if you want to really convince people.

According to the IPCC: https://ahdb.org.uk/carbon

Agriculture makes up 8.5% of global emissions - knock 75% of that off if we somehow convinced the *entire planet* to go vegan, add a bit back for the extra veg we need to grow and other unknown inefficiencies and we're talking a relatively small shift in GHG emissions compared to other areas. We need to cut 50% total, not 5%!

Yes, every little bit helps... but as I emphasised above, you need the ENTIRE PLANET to switch to veganism in the next 5-10 years and it's still not enough. The serious danger of the rhetoric put out by vegan activists in regards climate change is that people will think they've "done their bit", when in reality it's peanuts and completely disregards the biggest factors in climate change.

The single biggest way to reduce our environmental burden isn't eating more veg/less meat, it's voting for green causes - both politically and with your wallets (investments, pensions, local goods, etc). By all means go veggie on top, but please don't do just that!