As per golang-nuts:

I am trying to get vendoring working with submodules which is supposed to be working according to s/go15vender. The building part works fine but the "go get" part doesn't.

To reproduce I created a test repo by doing the following:

Created a test package and added a submodule via:

 git submodule add https://github.com/dgrijalva/jwt-go vendor/github.com/dgrijalva/jwt-go

Committed everything.

Now, if I do:

GO15VENDOREXPERIMENT=1 go get github.com/StephanDollberg/govendorsubmodulestest

I get the error:

# cd .; git --git-dir=/$GOPATH/src/github.com/StephanDollberg/govendorsubmodulestest/.git submodule update --init --recursive
No submodule mapping found in .gitmodules for path 'vendor/github.com/dgrijalva/jwt-go'
package github.com/StephanDollberg/govendorsubmodulestest: exit status 1

The interesting thing is that if I do a go get -u after that it checks out the submodule correctly. Looking at the error and the source of the go command I think it has to do with --git-dir not working together with the submodule command.

I tested this on Windows, Mac OSX and Linux (all Intel 64bit) with go 1.5.1 and several git versions (2.3, 2.4, 2.5).