I think the example there is hard to follow. Perhaps with more standard package names, such as golang.org/x/net or a popular github.com package, instead of the made up foo, baz, quux and so on it would be easier to understand exactly what advantages vendoring gives you.
Even if one vendors the external dependencies, the code that uses them must still be on GOPATH or the compiler would not find them. This is at least with go 1.6.
https://golang.org/cmd/go/#hdr-Vendor_Directories
I think the example there is hard to follow. Perhaps with more standard package names, such as golang.org/x/net or a popular github.com package, instead of the made up foo, baz, quux and so on it would be easier to understand exactly what advantages vendoring gives you.