You review it like it wasn't AI generated. That is: ask author to split it in reviewable blocks. Or if you don't have an obligation to review it, you leave it there.

This is it. The fact that the PR was vibe coded isn't the problem, and doesn't need to influence the way you handle it.

Eh, ask the author to split it in reviewable blocks if you think there's a chance you actually want a version of the code. More likely if it's introducing tons of complexity to a conceptually simple service you just outright reject it on that basis.

Possibly you reject it with "this seems more suitable for a fork than a contribution to the existing project". After all there's probably at least some reason they want all that complexity and you don't.