Within the narrow framework of what is consumed to sustain our lives, vegans do make superior ethical choices. Their diets consume less natural resources and decreases the suffering of sentient beings.
However, if you can argue that omnivores are equally or more ethical in their food choices, I'd be happy to entertain the notion.
Please keep in mind that you are an omnivore. You may be trying to convince yourself that you - and everyone else - are herbivorous, but it simply isn't so.
This idea about consuming less natural resources and decreasing the suffering of sentient beings: How far do you wish to extend that idea? Rice paddies expel huge amounts of methane, a gas that is 23 times more efficient than CO2 at trapping atmospheric heat. We're already seeing how global warming is beginning to apply its stress points upon life forms.
Corn? Corn fields? Huge expenditure in water and fertilizer to keep them productive.
If you're going to turn 500 acres of wild land into 500 acres of cultivated crop such as corn, wheat, beans, peas, apples, potatoes, grapes, cabbages - you first have to displace the entire ecosystem within that 500 acres. Which means a diversity of plant life, fungi, bacteria, worms, insects, birds - all the way up to "sentient" beings such as rodents, foxes, wolves, deer, bears or whatever are going to be destroyed, or displaced.
When you plant your crop, you know that insects and birds and rodents are going to want their fill. You have to stop them. Cause them some suffering, even death, because the food is right there but you're going to take measures to deny them that food. Insects and birds and rodents are animals - where do we draw the line on their sentient-ness? Folks who know of crows, for example, can attest to their high intelligence, their cleverness - their ability to remember and correlate meaning.
I don't believe there's any delicate method to feed ourselves on this planet - you're going to adversely affect something, vegan or no. Ethics? All depends upon one's point of view - and how far back one wishes to step during the viewing. You'd have to travel far and wide to find fellow omnivores such as yourself - but who include meat in their diets (the vast majority of humanity) - to favor deliberately cruel treatment of animals in the food production cycle. Therefore you need to find those odd, few individuals and preach ethics to them.