When I search "amoral" in general I seem to only find results about one taking a completely amoral position, nihilistic like, of all ethics/morality. If I search "amoral" with "vegan" or "veganism" I still do not find material actually discussing the position. The closest content I can find is this Quora post that only mentions veganism and amorality.
I see the difference between veganism and not as boiling down to two fundamentally different beliefs. I'm talking about ideology (not reasons behind having a vegan diet: environmental, health, religious, etc.). That is, one either believes that it is morally wrong to kill other animals (more or less sentient animals). Or, one believes killing an animal is not wrong; it is an amoral action.
I am strictly talking about slaughter--not about inflicting pain or suffering. (Slaughter can be carried out virtually painlessly.) I do not equate suffering and slaughter even though they both fall under the word "harm."
Is this position defensible? Is it missing something? The social contract, I think, falls under the idea that killing an animal, in and of itself, is amoral. Although, I don't think it's needed for a decent position against veganism (i.e., slaughtering or painlessly exploiting an animal is wrong).
Arguing from a consistent position against veganism can be a bit tricky. On one side there is Scylla as speciesism. On the other there is Charybdis as taking a 'psychopathic' position (e.g., we should kill/eat mentally disabled people too).
I do not mean plausible intelligent alien life when referring to non-human animals. Let's just keep it simple and take them to be three animals: cows, chickens and pigs.