I wonder what experts think of this other option: going fully native for one platform then vibe-porting it to others.

It's never "just working", you'll still have to debug the vibe-ported apps. And you won't be good at it if your devs don't actually know the target platform.

All that to say, I don't think that vibe-porting will give you more than cross-platform. With cross-platform, at least your engineers understand the codebase.