There has been a lot of back and forth and discussion on my first post: Amazon vs Wattpad.
In the meantime the blog: The Passive Voice: A Lawyer's thoughts on authors, self-publishing and traditional publishing
has put up a long article on Kindle Vella, republishing their own information, including terms, royalties etc. FAQ's, guidelines etc. etc.
and at the end observations both by authors and a couple people weighing in.
I would like to add this:
I am a published (non-fiction) author with an established publishing house. (book is out of print) Standard industry pay for an author of a hardcopy or paperback book is 15 to 30%. That's to the author. Amazon's pay to an author is much better. I speak from personal experience. I loved my publisher, hated my paychecks which, by the way, only came in quarterly. I'm not sure how quickly Amazon pays.
There will be, no doubt a lot of updating on the subject. You might want to bookmark this blog, I'm sure he'll have more to say.
https://www.thepassivevoice.com/kindle-vella-royalties-content-guidelines-and-publishing-process/
P.S. Make sure to read the "about" on the author/attorney. He states clearly that everything in his blog may not be pertinent to your situation and is for discussion and should not be construed as legal advice. But it does give plenty of information.