For core business logic you aren't vibing up entire apps at once. You're still making carefully considered changes of about the same size you did before, but now it takes 15 minutes instead of 15 hours. You still need a pretty good understanding of what you want ahead of time, which makes it easy to check that what it did is sane. There's always that one thing you don't think of until 80% through the implementation.....you now find those corner cases immediately. Plus, you can generate tests really easily, which at least for me used to be the slow part. And those are easy to sanity check. And if there's something you don't like or don't understand, ask the AI about it.