I found vendor directories to be documented in a confusing way, so I'm not surprised.

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.