Veganism is the practice of not eating, wearing, or using anything that comes from an animal, and is sometimes referred to as "hardcore vegetarianism," and it has some perks.
As illustrated by Todd Ingram in Volume 3, being a vegan apparently gives one intense psychic powers if one focuses and dedicates one's self to being a vegan for long enough. These powers are insanely powerful and apparently strong enough to practically turn a person into a superhuman. The person must be strong-willed to make full use of these abilities (which Todd is not) or risk being over-whelmed by their own subconscious. Whether these powers are latent in every person, or given to vegans by a third party after proving their true devotion is up for debate as it appears one can still eat non-vegan food (like gelato and chicken parmesean) and use the full abilities until caught by the Vegan Police.
Rules, Violations, and Punishments
You need to be a vegan or else. The mechanics behind this aren't completely (or clearly) explained, though Todd makes note that most people only use a small fraction of their brain because the rest is filled up with "curds and whey." If it were an issue of the brain being filled with curds and whey as Todd suggests, then anyone lactose intolerant might have an easy time gaining vegan powers simply by abstaining from all dairy products. Although, lactose intolerant people still eat meat so they cannot gain mental abilities. Eating Chicken Parmasean or Gelato ONCE when one has been vegan for years isn't going to make a difference. It seems to suggest that one is given vegan powers from vegan academy, both of which Todd attended.
The rules for keeping one's vegan powers appear to be regulated by a third party known as the Vegan Police, and if one violates the vegan rules by eating dairy products as demonstrated by Todd, they can be stripped of their abilities. As the vegan police put it, "No vegan diet, no vegan powers." Upon breaking the Vegan code, the Vegan Police arrive on the scene and use a "de-veganizing ray" which is fired from their fingertips. It appears to be a widespread beam near impossible to dodge and renders the power-user's abilities inert after a single blast.
Despite all this, the only non-vegan psychic is Mobile, who, according to Wallace, is psychic "for his own reasons", which makes it likely that Todd either doesn't understand the truth behind his Vegan powers or he was just making up a bunch of bullroar.
The Vegan Shepherd's Pie Recipe
In Volume 2, Scott Pilgrim vs. The World, Scott, his friends, and Ramona Flowers all hang around one night and make a vegan shepherd's pie. While none of them are vegan, Scott claims they make it vegan to be "all-inclusive" since anyone can eat it. The recipe is provided in the book by Stephen Stills who works as a cook at The Happy Avocado Vegetarian Restaurant.