VFE - The Vegan Friend Effect
A vegan amongst a group of people will be the deciding factor in where they all eat together. For every vegan served, several more non-vegans eat at the restaurant because of them.
Maybe vegans are few, but they have omnivore friends. When it comes to choosing a restaurant, the vegan in the group is going to say, "let's go to Pizza Express now it does dairy-free cheese and Cheatin' pepperoni!". They sell one vegan pizza but may also sell 10 standard pizzas because of it.
Only the manager on the ground can see this, as the crude numbers sent head office fail to show up the "vegan friend effect". But it's there and it makes a huge impact.