I heard about it when it launched, but it doesn't seem to have made much of an impact. I think it was meant to compete with dedicated websites for serialised fiction, like WebNovel and Dreame, and maybe even Wattpad. But you had to pay for episodes with some Amazon-only currency that you couldn't use for anything else, not even other stuff that Amazon sells.
Amazon didn't make it very appealing to writers, either. I write fiction (as a hobby-that-sometimes-pays-for-dinner-or-beers). Amazon might've relaxed the rules since I looked at Vella, but I think I would've had to write something specifically for Vella. They wouldn't accept anything that had been published anywhere else, not even on their other exclusive platform, Kindle Unlimited. (That one's been a runaway success. Maybe they thought it would be easy to replicate with a different reading model.) The Amazon-only money made it hard to work out how much I'd get paid for each chapter or story that a reader read. I didn't bother looking into whether I'd be allowed to publish a Vella-specific story somewhere else after some time. The idea of having to write exclusively for an as-yet-unproven market was enough of a deal breaker on its own.
More telling, perhaps - I'm in various writers' groups on Facebook, with many members who are much more successful than me, and I don't think I've ever seen anyone mention Vella as a viable marketplace for stories.