[calcifer]

> in module mode, using the vendor directory exclusively

But that's not what I'm asking. I want "go build" (with some flags) to use my fork (vendored if needed) for one library, and pick up all other dependencies from the module cache as usual. How do I that?

Because this is already possible in a vendor-only world; I just edit the code in vendor.

[EdiX]

You check out the dependency you want to change and add a replace directive in go.mod. Either a temporary one to a file system path or a permanent one to a forked repository.