/snark
I'd think of a Next-driven BE as a BE for FE. And since it's co-located, You can share types, validators, utilities, (mental) models, etc. across the same codebase.
A technique that does have its place. Not always. Maybe not even half the time. But cleverer people than me use it to some effect.
Vercel itself I believe runs on a combination of Google Cloud and AWS.
Ideally I just get an endpoint from my backend-as-a-service (Supabase style) I can pop into Vercel as an environment variable, and use it as well in my moblile apps, public api, etc.
I had used both proprietary & open-source Parse until 4 years back for several production applications but has since gone back to relational DB(pgSQL), so don't know the current status of the ecosystem.
And Vercel itself allows you to install backend services such as Redis caching, databases, or queues that you can pull in from those functions.