I believe that the details of the Vaishnava diet are not solely determined by the vegetarian principle of ahimsa (non-violence). The principles of ahimsa intersect with another set of principles, those of śaucam (cleanliness) to produce the various specific morays of the Vaishnava diet. The Vaishnava eats what s/he offers to Viṣṇu – and what s/he selects to offer to Viṣṇu is chosen from a menu that meets principles of non-violence as well as cleanliness. Thus, various Vaishnava groups don’t eat some foods, even though those foods are not intrinsically violent (eggs, onion, garlic, carrots, potatoes, mushrooms). Without considering the principle of cleanliness – only considering non-violence, this baffles the onlooker. Foods that do not grow in the light (onions, garlic, carrots, etc.) are “unclean.” Eggs are unclean because they are menstrual byproducts.
We find that the non-violent principles are far more important than the cleanliness principles, however. Because many pure Vaishnava’s will offer and eat foods that do not grow in the light. In ISKCON, for example, carrots are frequently eaten. There is room for leeway in the cleanliness principle, but not nearly as much room for it in the non-violence principle.