Different company. We evaluated Vitess in comparison to ProxySQL. While Vitess was very promising, it was a large refactor and an all or nothing proposition to switch to it. ProxySQL allowed us to move to it piecemeal and adopt a crawl, walk, run solution in adopting it.
Interesting, didn't realize it imposed a lot of client side changes, I'm curious to hear your experience on that? Was it the change in MySQL compatibility that makes it an upfront cost?
With Vitess I believe most get started with just deploying VTGate and just proxying their existing queries to their existing MySQL server, then once that is working nicely, start to range shard and expand the cluster sitting behind VTGate.
It was several years ago and I don't recall the details. And it is always possible we missed something or that things have changed on the Vitess side. I do recall the impression that we would have to have a "cut over" and it would be going from zero queries to full load. It seemed doable but leadership decided the risk was too high.