Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.10

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Darwin

What did you do?

Ran vgo build on a package that imports gopkg.in/alecthomas/kingpin.v3-unstable.

What did you expect to see?

Success.

What did you see instead?

Lack of success:

vgo: import "github.com/alecthomas/substrate/cmd/substrated" ->
	import "gopkg.in/alecthomas/kingpin.v3-unstable": invalid gopkg.in/ path: "gopkg.in/alecthomas/kingpin.v3-unstable"