What follows is based on the work of Ayn Rand, summarised in her essay, The Objectivist Ethics. Some definitions and statements there are closely rephrased.
Ethics are a code of values to guide human choice and action.
Their purpose is to help people live, to provide them a basic framework for a successful life.
Like science, ethics is based on metaphysics: nature, what exists, what is.
Ethics (or morality) exists independently of what humans think. As in science, the goal is to discover what the rules of ethics are—not to ‘invent’ them, at least so far as they support life.
As such, ethics are an objective, metaphysical necessity of life.
The hardness of a rock is is a metaphysical fact, and that hardness is judged according to a scale of hardness.
You can’t wish the rock and its hardness away. It will continue to exist and have the characteristic of hardness regardless of your wish.
Likewise, the speed of a plane is a metaphysical fact, and it’s speed is judged according to a scale of speed.
You can’t wish the plane away and try to measure its speed according to a scale of taste.
Similarly, the appropriate measure for life is a scale of life. Because it’s alive, constantly changing and complex, though, it’s typically measured in an order rather than a numerical value such as 53.85. (Although some markers of physical health are measured this way eg blood pressure.)
All the same, as Ayn Rand pointed out, life is the standard of value: whatever furthers it is good, whatever harms it is bad.
This standard applies to all life, whether plant, human or other animal.
The difference in applying that standard lies in the consciousness of life and its practical relation to other life (the context that beings find themselves in).
Even so, veganism is an integral part of a consistent moral code.
While ethics are a human invention, an explicit code for people to live by, the code relates to everything, whether alive or not, human or not.
While people may behave ethically without conscious knowledge of this code, often they don’t, defying metaphysical requirements of life, resulting in harm, whether minor or on a huge scale.
A sound ethical code makes the rules for living explicit, is accurate—ie it supports life rather than destroys it—and is based on metaphysics, what exists in fact, rather than fantasy.