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https://github.com/golang/go/wiki/Modules#how-do-i-use-vendoring-with-modules-is-vendoring-going-away

How do I use vendoring with modules? Is vendoring going away?
The initial series of vgo blog posts did propose dropping vendoring entirely, but feedback from the community resulted in retaining support for vendoring.

In brief, to use vendoring with modules:

go mod vendor resets the main module's vendor directory to include all packages needed to build and test all of the module's packages based on the state of the go.mod files and Go source code.
By default, go commands like go build ignore the vendor directory when in module mode.
The -mod=vendor flag (e.g., go build -mod=vendor) instructs the go commands to use the main module's top-level vendor directory to satisfy dependencies. The go commands in this mode therefore ignore the dependency descriptions in go.mod and assume that the vendor directory holds the correct copies of dependencies. Note that only the main module's top-level vendor directory is used; vendor directories in other locations are still ignored.
Some people will want to routinely opt-in to vendoring by setting a GOFLAGS=-mod=vendor environment variable.
Older versions of Go such as 1.10 understand how to consume a vendor directory created by go mod vendor, as do Go 1.11 and 1.12+ when module mode is disabled. Therefore, vendoring is one way for a module to provide dependencies to older versions of Go that do not fully understand modules, as well as to consumers that have not enabled modules themselves.

If you are considering using vendoring, it is worthwhile to read the "Modules and vendoring" and "Make vendored copy of dependencies" sections of the tip documentation.