I've seen authors who stand anywhere between "I'll give this a try and see how it goes" and "I'm not touching it". On my part, I've emailed Amazon pointing out the worst parts of the terms and am gonna be waiting a while to see what they change and don't change before release. Not in the US, so couldn't jump the bandwagon early, anyway.
What didn't inspire much confidence was looking at readers' reactions and seeing most of them saying they were not going to touch Vella. Granted, it was reddit and everyone on reddit is just a big ol' grump, but their reasoning seemed to be tied more to the fact that the app will be iOS / Android and not on Kindle itself, where they prefer reading, which is a somewhat of a fair criticism. If the idea was to tap into Amazon's readers for this, there might need to be a lot more rethinking in the works, including a new Kindle that supports Vella (which people were asking for and seemed on board with getting, Amazon just needs to deliver; heck, they may have already foreseen it and have it in the works).
But, for now, it all looks a lot like "Amazon sees Kakao and Naver buying up the English speaking webnovel market and wants to get their share before that happens", which... Might not turn out great, as everything seems so hastily put together and both of those other companies have been into the serialized fiction business for years and are giants in it. But that's a whole other thread to be made. Another good article to read is this one!