Why would that be poetic justice? Vegans are contributing to a cleaner, healthier, less cruel planet (and by the way, can have perfectly healthy diets without the increased risk of heart disease, cancer, etc., that meat-eaters have). Meat-eaters are doing the opposite -- hence poetic justice in experiencing a health problem after supporting the torture and slaughter of others.

How is "acting like that" -- i.e., standing in opposition to the negative status quo -- in any way equivalent to supporting extreme suffering?