The vast majority of living matter is plants, not animal, therefor the vast majority of oil derives from plants.

That said, this is based on a fundamental misunderstanding of what veganism is. Veganism is about avoiding animal "exploitation" to whatever extent possible. Being turned into oil after dying in tar 100 million years ago has nothing whatsoever to do with human exploitation of animals.

I hear you, but whenever I hear the phrase "to whatever extent possible", it seems like a cop-out for "to whatever extent I deem convenient". If all of a sudden some component of veganism becomes too inconvenient, would they just change the rules?

"Possible" isn't really that weak of a condition. You'd have to be very disingenuous to interpret that as "convenient". I don't know why anyone would do that, since there aren't any social advantages to being vegan.

If there were some sort of mandate to be vegan, then I'm sure people would invoke all manner of excuses. That's not the world we live in, though.

"Possible" is a really strong condition. The reason I replace it with convenient is because I don't think anyone takes the word "possible" seriously.

It's possible to live in a world without plywood, glue, and dyes derived from animals, but I don't know any vegans who are willing to go that far. It's just not very convenient.

Vegans that are vegans for ethical reasons aren't opposed to animals dying naturally and their bodies finding other uses. They're against killing animals that would otherwise be alive.

Are they ok with using leather, furs, or something like an ivory tusk from animals that died naturally? I thought those kinds of animal by-products were against the rules no matter what

Probably, assuming they felt confident that they died naturally.

Let us remember that these people, by not eating meat, do not usually think clearly.