cd thirteen
! go list -deps
stderr 	'(Go version in go.mod is 1.13, so vendor directory was not used.)'

cd ../unspecified
! go list -deps
stderr 	'(Go version in go.mod is unspecified, so vendor directory was not used.)'

-- thirteen/foo.go --
package foo

import _ "github.com/foo/bar"
-- thirteen/go.mod --
module example.com

go 1.13
-- thirteen/vendor/github.com/foo/bar/bar.go --
package bar
-- unspecified/foo.go --
package foo

import _ "github.com/foo/bar"
-- unspecified/go.mod --
module example.com
-- unspecified/vendor/github.com/foo/bar/bar.go --
package bar