I would argue that human meat is vegan no matter if consent was given, as long as
- the human was not intentionally killed (and the meat is taken after death), and
- the human was free (i.e., not a slave).
This applies to non-human animals, too: it would be vegan to take the meat of animals who died in the wild (without human influence) or of animals who where accidentally killed e.g. on the road.
Without consent, taking/using such meat might be unethical for other reasons (e.g., post-mortem rights, extrinsic moral value given by others), but these have nothing to do with veganism. If consent was given, these other reasons no longer apply, and taking/using the meat would be ethical.
Meat from a human who still lives is only vegan if
- the human gives consent to the injury/amputation (or it’s in their best interest), and
- the human gives consent to the use (e.g., eating) of the body part.
This mostly can’t apply to non-human animals, as they can’t give consent. An exception would be if an amputation is medically required (i.e., it’s in the animal’s best interest), and the animal has no need for the amputated body part.
Based on the Vegan Society’s definition, something is vegan unless it involves exploitation or cruelty.
If you are dead, you are non-sentient (→ you no longer have intrinsic moral value), so nothing can be exploitative/cruel anymore.
If you give true consent to an action that would otherwise be exploitative/cruel, that action is no longer exploitative/cruel.
Analogous examples are post-mortem and living organ donations:
It’s vegan to take the heart from, and accept the heart of, a dead person (but possibly unethical if the person didn’t agree to it during their lifetime).
It’s vegan to take the kidney from, and accept the kidney of, a living person who agreed to it.