We're not talking about consensual exploitation. Were talking about behaviors that aren't exploitation due to, or perhaps shown not to be so by consent. There's no need to explicitly mention consent because a) it would needlessly complicate the definition, b) as a practical matter, it almost never actually arises except in these sorts of thought experiments, and c) it's already included implicitly in the concept of exploitation.

Let's look at our original thought experiment: "It's vegan to eat someone who has consented to being eaten."

Usually we don't put too much thought into this sort of stuff because it doesn't really come up much outside of tongue in cheek mention, but I digress.

OK, so off the bat, if you think about it, there are indeed some problems with this statement. There could be systemic issues that made them consent to something harmful because the transactional benefit outweighs the harm to them. So in that sense, you're right, looking directly for exploitation is the more objectively vegan thing to do.

However what if they have a genuine desire to be eaten (non-injuriously or posthumously, hopefully) where there are no confounding influences like above? The absence of exploitation is indicated through consent, in this case, and indeed, without any form of consent the other party would have no way to know of their desire to be eaten.

I think maybe a more realistic example than eating someone would be "Is it vegan to honour someone's organ-donor card?" That seems to me to be a fairly clearcut case of accepting consent as implying non-exploitation.